17

Chordata: Urochordata

and Cephalochordata

Chapter Outline 17.1 Evolutionary Perspective

Phylogenetic Relationships 17.2 Phylum Chordata

Subphylum Urochordata Subphylum Cephalochordata

17.3 Further Phylogenetic Considerations

These tunicates (Clavelina picta, subphylum Urochordata) look nothing like

members of the phylum Chol'data, but they are! Careful examination of their

larval stage reveals all the features that al'e hallmal'ks of the phylum. 11,e pail'

of openings seen on each tunicate are siphons, which are the openings for

incurrent and excurrent water that ci1·culates thmugh this filter-feeding animal.

1 7 .1 EVOLUTIONARY PERSPECTIVE

LEARNING OUTCOME

1. Describe the evolutionary relationships of the Chordata to other animal phyla.

Some members of the phylum discussed in this chapter are more familiar to you than members of any other group of animals. This familiarity is not without good reason, for you yourself are a member of this phylum-Chordata. Other members of the Chordata, however, are much less familiar. During a walk along a seashore you could see animals clinging to rocks exposed by low tide. At first glance, you might describe them as jellylike masses with two openings at their unattached end. Some live as solitary individuals; others live in colonies. If you handle these animals, you may be rewarded with a stream of water squirted from their openings. Casual observations provide little evidence that these small filter feeders, called sea squirts or tunicates, are chordates. However, detailed studies have made that conclusion certain. Tunicates and a small group of fishlike cephalochordates arc often called the invertebrate chordates because they lack a vertebral column (figure 17.1).

Phylogenetic Relationships

Animals in the phylum Chordata share deuterostome characteristics with echino­derms and hemichordates (see figure 16.19). Most zoologists, therefore, believe that ancestral representatives of these phyla were derived from a common deuterostome ancestor (see figure 17.1). As discussed at the end of chapter 16, pharyngeal slits are characteristic of Deuterostomia, and they play very important roles in the Chordata.

SECTION REVIEW 1 7 .1

Members of the phylum Chordata are derived from a common deuterosomate ances­tor along with the Echinodermata and Hemichordata.

What important character is shared by deuterostomes (but was lost early

in the echinoderm lineage)?

Chordata: Urochordata and Cephalochordata 321

r :';.;-.-.I •1Yl i ' . , II ,.

-: .... ••'•• ,.. _ :--·

Protists

FIGURE 17.1

Basal Phyla

Lophotrochozoa Deuterostomia

Evolutionary Relationships of Chordata to Other Animals. This figure shows one interpretation of the relationships of the Chordata to other members of the animal kingdom (see pages xvi-xvii). The relationships depicted here are based on evidence from developmental and molecular biology. Chordates are placed within the Deuterostomia along with the Echinodermata and Hemichordata. This tunicate, or sea squirt (Polycmpa aurata), is an invertebrate chordate that attaches to substrates in marine environments. Note the two siphons for circulating water through a filter-feeding apparatus.

17 2 I {:JI( RDA'IA

LEARNING OUTCOMES

1. Describe the five unique characteristics of members ofthe phylum Chordata.

2. Compare adult tunicates to the generalized chordatebody form.

3. Compare adult cephalochordates to the generalizedchordate body form.

Although the phylum Chordata (kor-dat'ah) (L. chorda, cord) does not have an inordinately large number of species (about 45,000), its members have been very successful at adapting to aquatic and terrestrial environments through­out the world. Sea squirts, members of the subphylum

Urochordata, are briefly described in the "Evolutionary Perspective" that opens this chapter. Other chordates include lancelets (subphylum Cephalochordata) and the vertebrates (subphylum Craniata) (table 17.1). Character­istics of the phylum Chordata include:

l. Bilaterally symmetrical, deuterostomate animals2. A unique combination of five characteristics present at

some stage in development: notochord, pharyngeal slitsor pouches, dorsal tubular nerve cord, postanal tail, andan endostyle or thyroid gland

3. Complete digestive tract4. Ventral, contractile blood vessel (heart)

The combination of five characteristics listed in number 2 is distinctive of chordates, and these characteristics are dis­cussed further in the paragraphs that follow (figure 17.2).

The phylum is named after the notochord (Gr. noton,

the back+ L. chorda, cord), a supportive rod that extends most of the length of the animal dorsal to the body cavity and into the tail. It consists of a connective-tissue sheath that encloses cells, each of which contains a large, fluid-filled vacuole. This arrangement gives the notochord some turgid­ity, which prevents compression along the anteroposterior axis. At the same time, the notochord is flexible enough to allow lateral bending, as in the lateral undulations of a fish during swimming. In most adult vertebrates, cartilage or bone partly or entirely replaces the notochord.

Pharyngeal slits are a series of openings in the pha­ryngeal region between the digestive tract and the outside of the body. As described previously, pharyngeal slits are also found in the Hemichordata. These slits are not unique to the chordates, but they are adapted for use in important and dis­tinctive ways in this phylum. In some chordates, diverticula from the gut in the pharyngeal region never break through to form an open passageway to the outside. These diverticula are then called pharyngeal pouches. The earliest chordates used the slits for filter feeding; some living chordates still use them for feeding. Other chordates have developed gills in the pharyngeal pouches for gas exchange. The pharyngeal slits of terrestrial vertebrates are mainly embryonic features and may be incomplete.

322 CHAPTER SEVENTEEN

TABLE 17.1

CLASSIFICATION OF THE CHORDATA

Phylum Chordata (kor-dat'ah) Occupy a wide variety of marine, freshwater, and terrestrial

habitats. A notochord, pharyngeal slits, a dorsal tubular nerve cord, a postanal tail, and an endostyle or th roid gland are all present at some time in chordate life histo­ries. About 45,000 species.

Subphylum Urochordata (u"ro-kor-da'tah) Notochord, nerve cord, and postanal tail present only

in free-swimming larvae; adults sessile, or occasionally planktonic, and enclosed in a tunic that contains some cellulose; marine. Sea squirts or tunicates.

Class Ascidiacea (as-id"e-as' e-ah) All sessile as adults; solitary or colonial; colony

members interconnected by stolons. Sea squirts (Ascidia, Ciona).

Class Appendicularia (a-pen"di-ku-lar'e-ah) (Larvacea) (lar-vas' e-ah) Planktonic; adults retain tail and notochord;

lack a cellulose tunic; epithelium secretes a gelatinous covering of the body. Appendicularians (Fritillaria).

Class Thaliacea (tal"e-as'e-ah) Planktonic; adults are tailless and barrel shaped;

oral and atrial openings are at opposite ends of the tunicate; muscular contractions of the body wall produce water currents. Salps (Salpa, Tbetys).

Subphylum Cephalochordata (sef'a-lo-kor-dat'ah) Body laterally compressed and transparent; fishlike; all

five chordate characteristics persist throughout life. Amphioxus (Branchiostoma).

Subphylum Craniata (kra"ne-ah'tah)• Skull surrounds the brain, olfactory organs, eyes, and

inner ear. Unique embryonic tissue, neural crest, contributes to a variety of adult structures including sensory nerve cells, and some skeletal and other connective tissue structures.

I,ih-aphylum H'yperotreti (lli"per--0\' 1· 't ) Fishlike; skull consisting of cartilaginous bars;

jawless; no paired appendages; mouth with four pairs of tentacles; olfactory sacs open to mouth cavity; 5 to 15 pairs of pharyngeal slits; ventrolateral slime glands. Hagfishes.

'M11ntl:ic.r• r,i llw ,i,1ai;it11 1,r• loibci11,:«sd In dlap1 •11, 11:l 1hrnugh 22.·

Infraphylum Vertebrata ( ver''te-bra' tah) Notochord, nerve cord, postanal tail, and

pharyngeal slits present at least in embryonic stages; vertebrae surround nerve cord and serve as primary axial support.

Class Petromyzontida (pet' ro-mi-zon"tid-ah) Fishlike; jawless; no paired appendages;

cartilaginous skeleton; sucking mouth with teeth and rasping tongue. Lampreys.

Class Chondrichthyes (kon-drik'thi-es) Fishlike; jawed; paired appendages and cartilaginous

skeleton; no swim bladder. Skates, rays, sharks.

Class Actinopterygii (ak"tin-op'te-rig-e-i) Bony fishes having paired fins supported by

dermal rays; basal portions of paired fins not especially muscular, tail fin with approximately equal upper and lower lobes (homocercal tail); blind olfactory saca; pneumatic sacs function as swim bladder. Ray-finned fishes.

Class Sarcopterygii (sar-kop-te-rig' e-i) Bony fishes having paired fins with muscular lobes;

pneumatic sacs function as lungs; atria and ventricles at least partially divided. Lungfishes, coelacanths, and tetrapods.'"'

Class Amphibia (am-fib' e-ah) Skin with mucoid secretions; possess lungs and/or

gills; moist skin serves as respiratory organ; aquatic developmental stages usually followed by metamorphosis to an adult. Frogs, toads, salamanders.

Class Reptilia (rep-til'e-ah) Dry skin with epidermal scales; amniotic eggs;

terrestrial embryonic development. Snakes, lizards, alligators.

Class Aves (a'vez) Feathers used for flight; efficiently regulate body

temperature (endothermic); amniotic eggs. Birds.

Class Mammalia (mah-ma'le-ah) Bodies at least partially covered by hair;

endothermic; young nursed from mammary glands; amniotic eggs. Mammals.

"�lr!Ltl • �pc:iktui-:, th,; r,,Ut>Wini, r�,lr cl � of l!Jtm1\!Jcl" ,hn�Jtl J JncJt,U\lj;J 1vithli't lllll' ;' k<Wl<!l)'gJI (&¥ iJJ!ap/1,r. IH t111tt /.r.JJ

The tubular nerve cord and its associated structures

are largely responsible for chordate success. The nerve cord

runs along the longitudinal axis of the body, just dorsal to the

notochord, and usually expands anteriorly as a brain. This central nervous system is associated with the development

of complex systems for sensory perception, integration, and motor responses.

A fourth chordate characteristic is a postanal tail. (A postanal tail extends posteriorly beyond the anal opening.)

Either the notochord or vertebral column supports the tail.

Neural tube

Postanal tail

(a) Endostyle or thyroid gland

FIGURE 17.2

Chordate Body Plan. The development of all chordates involves the formation of a neural tube, the notochord, pharyngeal slits or pouches, a postanal tail, and an endostyle or thyroid gland. Derivatives of all three primary germ layers are present. (a) Lateral view. (b) Cross section.

The fifth characteristic unique to chordates is the pres­ence of an endostyle or thyroid gland. An endostyle is present on the ventral aspect of the pharynx in tunicates, cephalochordates, and larval lampreys. It secretes mucus that helps trap food particles during filter feeding. In adult lampreys and other chordates, the endostyle is transformed into an endocrine structure, the thyroid gland. (See ''How Do

We Know about the Evolution of the Thyroid Gland from the Endostyle?" p. 325.)

Subphylum Urochordata

Members of the subphylum Urochordata (u"ro-kor-dah'tah) (Gr. uro, tail + L. chorda, cord) are the tunicates or sea squirts. The ascidians comprise the largest class of tunicates (see table 17.1). They are sessile as adults and are either solita1y or colonial. The appendicularians and thaliaceans are plank­tonic as adults (figure 17.3). In some localities, tunicates occur in large enough numbers to be considered a dominant life-form.

Sessile urochordates attach their saclike bodies to rocks, pilings, ship hulls, and other solid substrates. The unattached end of urochordates contains two siphons that permit sea­water to circulate through the body. One siphon is the oral siphon, which is the inlet for water circulating through the body and is usually directly opposite the attached end of the ascidian (figure 17.4). It also serves as the mouth open­

ing. The second siphon, the atrial siphon, is the opening for excurrent water.

Chordata: Urochordata and Cephalochordata 323

The body wall of most tunicates (L. tunicatus, to wear a tunic or gown) is a connective-tissue-like covering, called the tunic, that appears gel-like but is often quite tough. Secreted by the epidermis, it is composed of proteins, various salts, and cellulose. Some mesodermally derived tissues, including blood vessels and blood cells, are incorporated into the tunic. Rootlike extensions of the tunic, called stolons, help anchor a tunicate to the substrate and may connect individuals of a colony.

Maintenance Functions

Longitudinal and circular muscles below the body-wall epi­thelium help change the shape of the adult tunicate. They act against the elasticity of the tunic and the hydrostatic skeleton that seawater confined to internal chambers creates.

The nervous system of tunicates is largely confined to the body wall. It forms a nerve plexus with a single ganglion located on the wall of the pharynx between the oral and atrial openings (figure 17.Sa). This ganglion is not vital for coordinating bodily functions. Tunicates are sensitive to many kinds of mechanical and chemical stimuli, and receptors for these senses are distributed over the body wall, especially around the siphons. There are no complex senso1y organs.

The most obvious internal structures of the urochordates

are a very large pharynx and a cavity, called the atrium, that surrounds the pharynx laterally and dorsally (figure 17.Sb). The pharynx of tunicates originates at the oral siphon and is continuous with the remainder of the digestive tract. The oral

margin of the pharynx has tentacles that prevent large objects from entering the pharynx. Numerous pharyngeal slits called stigmas perforate the pharynx. Cilia associated with the stig­mas cause water to circulate into the pharynx, through the stigmas, and into the surrounding atrium. Water leaves the tunicate through the atrial siphon.

The digestive tract (gut) of adult tunicates continues from the pharynx and ends at the anus near the atrial siphon. The endostyle is a ventral ciliated groove that forms a mucous sheet (figure 17.Sb). Cilia move the mucous sheet dorsally across

the pha1ynx. Food particles, brought into the oral siphon with incurrent water, are trapped in the mucous sheet and passed dorsally. Food is incorporated into a string of mucus that by ciliary action moves into the next region of the gut tract. Digestive enzymes are secreted in the stomach, and most absorption occurs across the walls of the intestine. Excurrent water carries digestive wastes from the anus out of the atrial siphon.

In addition to its role in feeding, the pharynx also func­tions in gas exchange. Gases are exchanged as water circu­lates through the tunicate.

The tunicate heart lies at the base of the pharynx. One

vessel from the heart runs anteriorly under the endostyle, and another runs posteriorly to the digestive organs and gonads.

Blood flow through the heart is not unidirectional. Peristaltic contractions of the heart may propel blood in one direction for a few beats; then the direction is reversed. The significance

324 CIIAPTER SEVENTEEN

•• ..

. ....

(a)

FIGURE 17.3

��_m _____ _

Subphylum Urochordata. (a) Members of the class Appendicularia are planktonic and have a tail and notochord that persist into the adult stage. Oiko labradorensis is shown here. (b) The thaliaceans, or salps, are barrel-shaped, planktonic urochordates. Oral and atrial siphons are at opposite ends of the body, and muscles of the body wall contract to create a form of weak jet propulsion. Cyclosalpa is shown in this photograph.

of this reversal is not understood. Tunicate blood plasma is colorless :md contains various kinds of amoeboid cells.

Ammonia diffuses into water that passes through the pharynx and is excreted. In addition, amoeboid cells of the circulato1y system accumulate uric acid and sequester it in the intestinal loop. Pyloric glands on the outside of the intes­tine are also thought to have excretory functions.

Reproduction and Development Urochordates are monoecious. Gonads are located near the loop of the intestine, and genital ducts open near the atrial siphon. Gametes may be shed through the atrial siphon for external fertilization, or eggs may be retained in the atrium for fertilization and early development. Although self­fertilization occurs in some species, cross-fertilization is the rule. Development results in the formation of a tadpolelike larva with all five chordate characteristics. Metamorphosis begins after a brief free-swimming larval existence, during which the larva does not feed. The larva settles to a firm sub­strate and attaches by adhesive papillae located below the mouth. During metamorphosis, the outer epidermis shrinks and pulls the notochord and other tail structures internally

(b)

for reorganization into adult tissues. The internal structures rotate 180° , positioning the oral siphon opposite thP �rlhP­sive papillae and bending the digestive tract into a U-shape (see figure 1 7. 4).

Subphylum. Cephalochordata

Members of the subphylum Cephalochordata (sef'a-lo-kor­dah'tah) (Gr. kephalo, head+ L. chorda, cord) are called lancelets. Lancelets clearly demonstrate the five chordate characteristics, and for that reason they are often studied in introductory zool­ogy courses.

The cephalochordates consist of two genera, Branchiostoma (amphioxus) and Asymmetron, and about 45 species. They are distributed throughout the world's oceans in shallow waters that have clean sand substrates.

Cephalochordates are small (up to 5 cm long), tadpole­like animals. They are elongate, laterally flattened, and nearly transparent. In spite of their streamlined shape, cephalochor­dates are relatively weak swimmers and spend most of their time in a filter feeding position-partly to mostly buried with their anterior end sticking out of the sand (figure 17. 6).

Chordata: Urochordata and Cephalochordata 325

Endostyle ---­

Stigma-------

lncurrent siphon

Dorsal

Anus----

Ventral

----Stigma

-cut edge of

Oral (incurrent)

\siphon Atrial (excurrent) siphon

Tunic /' Pharynx with ---stigmas

Endostyle --

-- Intestine

Gonad Stomach

t ,___

FIGURE 17.4

�

, ' ..

(a)

Posterior

Atrium-----+--- --

Atrial opening

Stigmas

(b )

FIGURE 17.5

body wall

----+-Tunic

Pharynx

Endostyle

Ventral blood sinus

Mucous sheet

Internal Structure of a Tunicate. (a) Longitudinal section. Black arrows show the path of water. (b) Cross section at the level of the atrial siphon. Small l>l:1d( arrows show movement of food trapped in mucus that the enuostyle produces.

Tunicate Metamorphosis. Small black arrows show the path of water through the body.

How Do We Know about the Evolution

of the Thyroid Gland from the Endostyle?

The study of the development of one group of vertebrates, the lampreys, has provided

insight into the endostyle's evolu­tionary fate. In addition to producing mucus for filter feeding in invertebrate

chordates, the endostyle can bind the amino acid tyrosine. This is true in larval lampreys. When a lar val

lamprey metamorphoses to an adult and becomes a predator, the mucus­secreting functions of the endostyle become secondary, and the endo­style is transformed into the thyroid gland. The iodine-containing secre­tions of the thyroid gland regulate metamorphosis and metabolic rate. The development of the thyroid gland

of lampreys is widely believed to demonstrate the evolutionary events that led to the vertebrate thyroid

gland. The endostyle of vertebrate ancestors probably had both mucus­secreting and endocrine functions. With the evolution of jaws and a more active, predatory lifestyle, endocrine functions would have been favored.

326 CHAPTER SEVENTEEN

"-..:"---:=--- Segmentally arranged muscles

1�<"---- Notochord

�:�.:;-,,...--:--- Dorsal tubular

Midgut cecum / Atrium

/ Atriopore /

Ventral fin

Anus

FIGURE 17.6

nerve cord

Tall

Subpbylum Cephalochordata. Internal structure of ./Jrtmcbic;stonw l:.Unphioxus) shown in its partially buried feeding position.

The notochord of cephalochordates extends from the tail to the head, giving them their name. Unlike the noto­chord of other chordates, most of the cells are muscle cells, making the notochorcl somewhat contractile. Both of these characteristics are prohahly adaptations to burrowing. Con­traction of the muscle cells increases the rigidity of the noto­chord by compressing the fluids within, giving additional support when pushing into sanely substrates. Relaxation of these muscle cells increases flexibility for swimming.

Segmentally arranged muscle cells on either side of the notochord cause undulations that propel the cephalochordate through the water. Longitudinal, ventrolateral folds of the body wall help stabilize cephalochordates during swimming, and a median dorsal fin and a caudal fin also aid in swimming.

An oral hood projects from the anterior end of cepha­lochorclclte.s Ciliated, fingerlike projections. called cirri, hang from the ventral aspect of the oral hood and are used in feed­ing. The posterior wall of the oral hood bears the mouth open­ing that leads to a large pharynx. Numerous pairs of pha1yngeal slits perfornte the pharynx and are supported by cartilaginous gill bars. Large folds of the body wall extend ventrally around the pha1ynx and fuse at the ventral midline of the body, cre­ating the atrium, a chamber that surrounds the pha1yngeal region of the body. It may protect the delicate, filtering surfaces of the pharynx from bottom sediments. The opening from the atrium to the outside is called the atriopore (see figure 17.6).

Maintenance Functions Cephalochordates are filter feeders. During feeding, they are partially or mostly buried in sandy substrates with their mouths pointed upward. Cilia on the lateral surfaces of gill bars sweep water into the mouth. Water passes from the pharynx, through pharyngeal slits to the atrium, and out of the body through the atriopore. Food is initially sorted at the cirri. Larger materials catch on cilia of the cirri. As these larger particles accumulate, contractions of the cirri throw them

off. Smaller, edible particles are pulled into the mouth with water and a1e collected by cilia on the gill bars and in mucus secreted by the enclostyle. As in tunicates, the endostyle is a ciliated groove that extends longitudinally along the mid­ventral aspect of the pharynx. Cilia move food and mucus dorsally, forming a food cord to the gut. A ring of cilia rotates the food cord, dislodging food. Digestion is both extracellular and intracellular. A diverticulum off the gut, called the midgut cecum, extends anteriorly. It ends blindly along the right side of the pharynx and secretes digestive enzymes. An anus is on the left side of the ventral fin.

Cephalochordates do not possess a true heart. Contrac­tile waves in the walls of m::ijor vessels propel blood. Blood contains amoeboid cells and bathes tissues in open spaces.

Excretory tubules are modified coelomic cells closely associated with blood vessels. This arrangement suggests active transport of materials between the blood and the excretory tubules .

The coelom of cephalochordates is reduced, compared to that of most other chordates. lt 1s restricted to canals near the gill bars, the endostyle, and the gonads.

Reproduction and Development Cephalochordates are dioecious. Gonads bulge into the atrium from the lateral body wall. Gametes are shed into the atrium and leave the body through the atriopore. External fertilization leads to a bilaterally symmetrical larva. Larvae are free swimming, but they eventually settle to the substrate before metamorphosing into adults.

SF.CTrON REVIEW 1 7 .2

Members of the phylum Chordata are deuterostomates that possess five distinctive characteristics at some stage in devel­opmt'nt: notochorcL pharyngeal slits or pouches, dorsal tubular nerve cord, endostyle, and postanal tail. Members of the subphylum Urochordata are the tunicates. They may be sessile or planktonic filter feeders, and they reproduce through external fertilization, the development of a tadpole­

like larva, and metamorphosis to the adult. Members of the subphylum Cephalochordata burrow in sandy marine sub­strates, are filter feeders, and reproduce through external fer­tilization and the development of free-swimming larvae.

How are the five distinctive chordate characteristics

represented in tunicates and cephalochordates?

17.

C

F T

LEARNING OUTCOMES

l�OGEN •.TIC

1. Describe the relationships between members of thethree chordate subphyla.

2. Characterize members of the largest craniate infra phy­

lum, Vertebrata.

Even though the evolutionary pathways leading from the common deuterostome ancestor to the Echinodermata, Hemi­chordata, and Chordata are speculative; evidence linking these phyla is well established (figure 17.7). Recent evidence suggests that the use of pharyngeal slits in filter feeding is a very old deuterostome characteristic.

All molecular, morphological, and developmental evidence point to the chordates as a monophyletic clade. The third chordate subphylum is Craniata (see table 17.1 and figure 17. 7). The origin of the chordate clade is con­troversial (see Evolutionary Insights, page 328) as is the relationship of the tunicates, cephalochordates, and crani­ates. Some genomic DNA evidence supports a sister-group relationship between the tunicates and the craniates. Studies of rRNA genes support a sister-group relationship between

Ambulacraria

Hemichordata

Chordata: Urochordata and Cephalochordata 327

the cephalochordates and the craniates. Evidence for the latter seems to be mounting, thus the representation in figure 17.7.

The largest and most successful craniates belong to the infraphylum Vertebrata. Bony or cartilaginous vertebrae that completely or partially replace the notochord characterize the vertebrates. The development of the anterior end of the ne1ve cord into a three-part brain (forebrain, midbrain, and hind­

brain) and the development of specialized sense organs on the head are evidence of a high degree of cephalization. The skeleton is modified anteriorly into a skull or cranium. There are eight classes of vertebrates (see table 17.1). Because of

their canilaginous and bony endoskeletons, vertebrates have left an abundant fossil record. Ancient jawless fishes were common in the Ordovician period, approximately 500 million years ago. Over a period of approximately 100 million years, fishes became the dominant vertebrates. Near the end of the Devonian period, approximately 400 million years ago,

Chordata

Buccal apparatus

----- Pharyngeal

---- Sessile and colonial

Diffuse epidermal nervous system ----'I. Tripartite coelom -- -­

Larvae with ciliary bands -----,.,.,

Pharyngeal slits

basket

Endostyle or thyroid gland

----Tadpole larva --- Postanal tail

Notochord

Dorsal tubular nerve cord

Radial cleavage, enterocoelous coelom formation

FIGURE 17.7

Enlargement of the neural tube forms a three-part brain

Endoskeleton including

One Interpretation of Deuterostomate Phylogeny. Developmental and molecular evidence link the echinoderms and hemichordates into the clade Ambulacraria. The Enteropneusta (*) is probably not monophyletic. The dorsal tubular nerve cord, notochorcl, postanal tail, and endostyle (thyroid gland in adult vertebrates) are important characteristics distinctive of the Chordata. Some of the synapomorphies that distinguish the chordate subphyla are shown. Molecular evidence regarding the relationship of the tunicates, cephalochordates, and craniates is contradictory.

EVOLUTIONARY iNSIGHTS

Early Deuterostome Evolution

T he rnigin or the dcurcroscome lineage i an unresolved question. The fact rh:u mesoderm forms while thi:: ccie­lom is forming (seej7gure 7.13) sugge.,t� to some z lo­

g1sts that this lineage was derived from cliploblastic ancestors. (In the protostomes, the coelom forms from preformed mesoderm, suggesting triploblastic ancestry) A hypothetical. bilateral, diplo­hlastic animal simil:1r to the planula larva of cnidarians has been suggested as a common ancestor of all cleulerostomes. The echi­noderms originmecl very P,1rly in this lineage-the earliest fossils that are truly echinoderm arc found in early Cambrian fossil beds.

\'(I, Garslang suggested that the origin of the primitive chor­dates may have occurred when ciliated bands present on most deuterostome larvae shifted dorsally to form the neural tube. Some modern ultrastruclural evidence suggests homology bet,veen embryonic ciiiary bands anci certain cells in the ner­vous system of amphioxus. The origin of pharyngeal slits ,Youlcl have promoted water movement and thus filter feeding and gas exchange. Garstang·s paedomorphosis hypothesis has been chal­lenged recently. Molecular data suggest that the free-swimming Appendicularia are more simil,ir to other chordates than the Asci­cliace;i whose larv;il stages have been implicatecl in the Garst;ing hypothesis. The oldest chordate fossils are from fossil beds in China ancl elate to about 530 million years before the present. Slightly younger (520 million years before the present) chordate fossils have been found in the Burgess Shale of British Columbia (box figure 17.1).

!'vlolecular evidence is providing cause for rethinking the rela­tionships between protostomes ancl deuterostomes. A nineteenth­century zoologist. Geoffroy Saint-Hilaire, studied the anatomy of a lobster from a ventral view. He noticed that, in this orientation, the ventral-to-dorsal sequence of nerve corcl, segmental muscles,

of these same structures in chordaces. He hypmhesized that Liie dorsal/ventral ;ixis in arthropods was homologous to, although inverted from, that of chordates.

This old hypothesis is being revived. Molecular biologists have discovered genes of fruit flies (Drosopbi/a me/anogaster) that con­trol the development of structures on the dorsal aspect of the fly ancl other genes that control the development of structures on rhe ventral aspect of Ll1e fly. Simih11 ly, there are genes that control

terrestrial vertebrates made their appearance. Since that time,

vertebrates have radiated into mo.st of the earth's habitats.

Chapters 18 through 22 give an account of these events.

Similarities between echinoderms and hemichordates war­rant their inclusion in the clacle Ambulacraria. These simi­

larities include shared molecular characteristics, the presence

of a unique tripartite coelom, and numerous developmt>ntal

Notochord

Segmented muscle

(a)

Dorsal

Brain Nerve Esophagus aorta Notochord Myomere

Endostyle Ventral Pharyngeal

(b)

aorta bar

BOX FIGURE 17 .1 Early Chordates. (a) This drawing is based on a fossil of a 520-million-year-olcl cephalochordate. Pikaia graci/e11s. from the Burgess Shale of British Columbia. (b) Haikouella is the oldest chordate fossil (530 million years old) found in fossil beds in China.

the development of dorsal and ventral structures in a frog (Xe110-

pus /aevis) and a fish (Dania rerio). Interestingly, the genes that control ventral development of the two chordates are very similar to the genes that control the dorsal development of the fly ancl vice ver5a. !D.jectior1 irlto �l frog e!11bryo of the n1f::'""PnePr RNA or prolein p1oducl� of the fly gene that controls dorsal develop­ment promotes the development of ventral features in the frog. The implication of this ,.._·ork is that the dorsal surface of cleutero­stomes may he homologous to the ventral surface of protostomes. Once again, molecular biology is providing a way of peering back into the evolutionary history of animals. In this case, it may be showing us one of the important changes that occurred at the divergence of the deuterostornatc lineage from other animals.

characteristics. The Chordata is a monophyletic clade that includes the subphyla Urochordata, Cephalochordata, and

Craniata. Contradictory evidence makes the exact relationships among these subphyla uncertain. Members of the infra phylum

Vertebrata are the largest and most successful chordates.

What evidence supports the clade Ambulacraria? What

evidence supports closer ties of the Hemichordata to the

Chordata? What hypothesis regarding hemichordate

affinity is better supported by the evidence you list?

SUMMARY

17 .1 Evolutionary Perspective

Echinoderms, hemichordates, and chordates share deutero­stome characteristics and are believed to have evolved from a common diploblastic or triploblastic ancestor.

17.2 Phylum Chordata

Chordates have five distinctive characteristics. A notochord is a supportive rod that extends most of the length of the animal. Pharyngeal slits are a series of openings between the digestive tract and the outside of the body. The tubular nerve cord lies just above the notochord and expands anteriorly into a brain. A postanal tail extends posteriorly to the anus and is supported by the notochord or the vertebral column. An endostyle functions in filter feeding or a thyroid gland functions as an endocrine organ.

Members of the subphylum Urochordata are the tunicates or sea squirts. Urochordates are sessile or planktonic filter feed­ers. Their development involves a tadpolelike larva.

The subphylum Cephalochordata includes small, tadpolelike filter feeders that live i.n shallow marine waters with clean, sandy substrates. Their notochord extends from the tail into the head and is somewhat contractile.

17.3 Further Phylogenetic Considerations

Echinoderms, hemichordates, and chordates comprise Deuterostomia.

Chordata is a monophyletic assemblage consisting of three subphyla: Urochordata, Cephalochordata, and Crani.ata. Cephalochordata is probably a sister-group to Craniata. The vertebrates are the largest and most successful craniates.

CONCEPT REVIEW QUESTIONS

1. Which one of the following would represent a valid sister­group for the phylum Chordata?

a. Ambulacraria c. Echinodermata

b. Hemichordata d. Enteropneusta

2. Which one of the following characters is distinctive of the Chordata but not necessarily unique to members of the phylum?

a. Dorsal tubular nerve cord

b. Postanal tail

c. Pharyngeal slits

d. Endostyle or thyroid gland

e. Notochord

Chordata: Urochordata and Cephalochordata 329

3. Members of the phylum Chordata possess all of the followingcharacteristics, except one. Select the exception.

a. Endostyle or thyroid gland

b. Dorsal tubular nerve cord

c. Postanal tail

d. Pharyngeal slits

e. Open circulatory system

4. Members of which of the following groups are planktonic asadults and possess a tail and notochord that persist into theadult stage'

a. Ascidiacea

b. Thaliacea

c. Appendicularia

d. Enteropneusta

S. The endostyle of urochordates was the evolutionary forerun-ner of the _____ of vertebrates.

a. thymus

b. pituitary gland

c. pancreas

d. thyroid gland

ANALYSIS AND APPLICATION

QUESTIONS

1. What evidence links echinoderms, hemichordates, and chor­dates to the same evolutiona1y lineage?

2. What evidence of chordate affinities is present in adult tuni­cates? In larval tunicates? If you only could examine an adult,would you be able to identify it as being a chordate?

3. Discuss the role of filter feeding in deuterostome evolution. Atwhat point i.n chordate evolution does feeding become forag­ing or predatory in nature?

4. Compare and contrast metamorphosis in the Echinodermataand the Urochordata. Do any similarities you describe indicatecommon ancestry of the two groups?

ZOOLOC.Y

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