The Protostome-Deuterostome Split The Protostome-Deuterostome Split

• The rest of the animal phyla have a true coelome. The rest of the animal phyla have a true coelome.

• These phyla are often refereed to as the "Eucoelmates", or simply Coelomates. These phyla are often refereed to as the "Eucoelmates", or simply Coelomates.

• The coelomate phyla can be divided into two distinct evolutionary lines. The coelomate phyla can be divided into two distinct evolutionary lines.

• Mollusks, annelids, and arthropods represent one of these lines collectively Mollusks, annelids, and arthropods represent one of these lines collectively called the protostomes. called the protostomes.

• The echinoderms and the chordates form the other line and are called The echinoderms and the chordates form the other line and are called deuterostomes. deuterostomes.

• These two groups of animals are distinguished by several fundamental These two groups of animals are distinguished by several fundamental differences in their development. differences in their development.

Early Development.

• Following fertilization, the zygote goes through a series of developmental steps termed cleavage.

• Cleavage begins with the first mitotic division of the zygote and ends when a hollow sphere of cells, the blastula, is produced.

• protostomes have determinate cleavage; begins differentiation early.

• the two daughter cells are different. Each cell to become one part of the body while the other cell makes up the remainder of the organism.

• deuterostomes have indeterminate cleavage. At the 2, 4 or 8 cell stage, the embryo can be split and each cell will act as a zygote and start the development process all over again. The fate of each cell or part of the organism has not yet been determined.

• In the protostome phyla, the opening (blastopore) becomes the mouth or the mouth and the anus.

In the deuterostome phyla, the blastopore becomes the anus and the mouth develops some time later at a different embryonic location.

The cavity formed becomes a true coelom. Because the coelom is produced as a result of splitting the mesoderm, these animals are also described as being schizocoelomates (schizos = split). The deuterostomes do not produce their coelom by splitting. Instead, the endoderm develops two pouches that extend from the archenteron into the blastocoel. These pouches eventually fill the blastocoel forming two new pouches, the coelom. The deuterostome animals are therefore termed enterocoelomates (enterous = gut).

Summary of Protostome-Deuterostome Dichotomy

Protostomes Deuterostomes

Spiral plane of cleavage Radial plane of cleavage

Determinate cleavage Indeterminate cleavage

Blastopore forms the mouth in adults

Blastopore forms the anus in adults

Schizocoelous coelom formation

Enterocoelous coelom formation

Protostome Phyla PHYLUM ANNELIDA The Segmented Worms. • There are approximately 9,000 species of segmented worms.

• The most familiar are the earthworms and the freshwater leeches.

• Two thirds of the phylum are much less familiar marine, polychaete worms. The principal characteristic feature of these animals is their external separation into repeating units or segments.

• Each segment is also termed a metamere.

• Annelids have a worldwide distribution. They are found in marine, freshwater and terrestrial habitats.

• In terrestrial environments, the worms cannot be removed from moisture for an extended period of time or they will dry up.

• Annelids are either herbivores or carnivores. The earthworms, for example, feed upon organic matter in the mud, while many of the marine forms are active hunters. The leeches are ectoparasites of vertebrate animals.

Phylum Characteristics: 1. Multicellular, bilaterally symmetrical, segmented

worms. 2. They are at the organ-system level of organization. 3. The annelids are triploblastic, schizocoelomate

protostomial animals.

4. With the exception of the leeches, each of the annelid worms has chitinous setae (chaetae) on their latero- ventral surface.

5. The coelom is separated at each segment by septa.

6. The digestive tract is complete.

9. Respiratory gas exchange is through the skin, gills or fleshy appendages, parapodia.

10. Blood is circulated through a closed circulatory system and the oxygen is transported on respiratory pigments. 11. The nervous system consists of a double ventral nerve cord.

12. The muscles of the body wall and intestinal tract pull against the hydrostatic skeleton of the coelom. 13. The segmented worms are hermaphroditic or with separate sexes. Fertilization is external and many produce trochophore larvae.

PHYLUM MOLLUSCA: • largest phylum (or division) of living organisms outside of the Arthropoda.

• includes snails, oysters, clams, squid and octopuses.

• There are approximately 50,000 species, 35,000 fossil species classified.

• range in size from microscopic to the giant squids

• primarily aquatic found in almost all marine and freshwater habitats, also live in terrestrial environments, avoid direct sunlit areas.

• wide variety of mollusks are used as food. Others are more harmful, damaging wooden ships and wharves (shipworms) and eating vegetable crops (slugs).

Chiton. Class Polyplicaphora.

Shipworm

Torsion in gastropods.

Class Bivalva

A scallop - a bivalve

Anatomy of a clam.

Class Cephalopoda

Some members of the class Gastropoda.

Phylum Characteristics: 1. These multicellular animals are bilaterally symmetrical or

develop into a form of asymmetry.

2. They have reached an organ system level of organisation. 3. The molluscs are triploblastic, protostomial schizocoelomates. 4. The ventral body wall is modified into a foot and the dorsal

body wall produces two tissue folds, the mantle.

5. They have a complete digestive system with extracellular digestion.

6. The outside of most of these animals is covered with a protective shell. The epidermis is usually ciliated with many mucous glands.

PHYLUM MOLLUSCA:

7. The coelom is reduced to a small cavity around the heart. 8. The open circulatory system has a heart, several major arteries, blood sinuses and respiratory blood pigments. 9. Gas exchange is across gills, lungs or the mantle wall. 11. The nervous system of most consists of 3 or 4 pair of ganglia, interconnecting fibers and sensory cells.

12. The muscles pull against the shell or a hydrostatic skeleton. 13. The animals are dioecious and reproduce only sexually.

PHYLUM ARTHROPODA: • named and as yet unnamed members of the phylum Arthropoda, there are most likely more of these animals than all other living organisms combined.

• The arthropods are, by any criteria, the most successful group of animals alive. As their name indicates, they are animals with `jointed appendages". Included in the phylum are spiders, scorpions, ticks, crabs, barnacles, ants, bees etc.

•Arthropods are found in all types of environments. They live in the ocean depths, high mountain peaks, deserts, tropical rain forests and in or on the bodies of other animals and plants.

• They obtain their food as carnivores, herbivores, omnivores and parasites.

• The largest arthropod is the Japanese crab (3.7m) and the smallest is a parasitic mite, (0.1mm).

• There is no group of animal that does more harm to humans and few that do as much good.

Phylum Characteristics: 1. The arthropods are multicellular, segmental,

bilaterally symmetrical animals. 2. They are triploblastic, protostomial

schizocoelomates. 3. Arthropods have reached the organ - system level

of organisation. 4. The body is typically separated into a head, thorax

and abdomen although one or more of these regions may be fused.

5. The arthropods have well developed striated and smooth muscles which pull against a cuticulate exoskeleton. The exoskeleton is made of lipid, protein and chitin and may become impregnated with calcium carbonate.

6. The coelom is usually reduced to a small haemocoel. 7. The digestive system is complete and digestion is extracellular.

8. The arthropods have an open circulatory system.

9. Respiration is by gills, book lungs and trachea. 10. Excretion is by green glands or malpighian tubules. 11. The nervous system is well developed, particularly the sensory component. Structurally it resembles the annelid nervous system.

12. Reproduction usually involves dioecious organisms. Fertilization is internal and development often involves one or more metamorphic changes.