20 ANIMALS CLASSIFYING ANIMALS

FISHES

Any study of the fishes shows that they are an immensely diverse array of animals.differing greatly in the range of habitats they occupy and their body forms andadaptations. As a consequence, most biologists regard the term "fishes," as aconvenient name, rather than a closely defined taxonomic entity, that describesaquatic vertebrates such as haqfishes, lampreys, sharks, rays, lungfishes, sturgeons,gars, and the advanced ray-finned fishes. There are a number of classification schemesfor the fishes but one of the most widely accepted recent ones recognizes five classesof living species and three classes that are now extinct. The five classes, whoseclassification is detailed below, are hagfishes, lampreys, cartilaginous fishes, lobe-finned fishes, and ray-finned fishes. These are grouped into two superdasses: jawlessfishes and jawed fishes. The three extinct classes are the pteraspidomorphs-jawlessarmored fishes, the jawed placoderms that were encased in bony plates, and theacanthcoiens, small true bony fishes with two long dorsal spines.

lAWLESSFISHES '!l~!!!~~"!!::'''''''~~!!::S::i::i~SuperclassAgnatha ' .......Lampreysarid haqfishes

JAWEDFISHES Supercla55 Agnatha. page 453Superclass Gnathostomata(includesall the groupsbelow)

CARTILAGINOUSFISHES"Chondrichthyes"ClassChondrichthyesSharks,rays,and allies

SubclassElasmobranchiiSharksRaysand allies

SubclassHolocephaliChimaeras

BONYFISHES"Osteichthyes"

ClassSarcopterygiiLungfishesand allies

ClassActinopterygii

SubclassChondrosteiBichirsand allies

SubclassNeopterygii

Divisionreieoste

Subclass Elasmobranchii, page 462

PrimitiveNeopterygii(garsand bowfin)

SubdivisionOsteoqlossomorphaBonytonguesand allies

SubdivisionElopomorphaEelsand allies

SubdivisionClupeomorphaSardinesand allies

Subdivision 05teoglossomorpha, page 472

SubdivisionEuteleostei(includesall the groups below)

SuperorderOstariophysiCatfish and allies

SuperorderProtacanthopterygiiSalmonsand allies

Superoroer StenopterygiiDraqonfishes and allies

Superorder CyclosquamataLizardfishesand allies

Supercrder ScopelomorphaLanternfishes

Supercrder PolymixiomorphaBeardfishes

SuperorderLampridiomorphaOpahsarid allies

Superorder ParacanthopterygiiCod, anqlerfishes. and allies

SuperorderAcanthopterygiiSpiny-rayedfishes

INVERTEBRATES

Over 95 percent of all animals are invertebrates. They are characterized by a structurethat they all lack: a backbone or vertebral column. Invertebrates are divided into about30 phyla, each displaying a distinct body form. Their evolutionary relationships can beinferred from their anatomy, their early development, and more recently frommolecular analyses, particularly DNA, the genetic code. Features that define phylainclude the organization of the body from a loose association of cells (Porifera),through tissue formation (Cnidaria) to the development of organs (Platyhelminthes)The acquisition of a fluid-filled body cavity was a defining point in animal evolutionthat allowed animals, such as Nematoda, Annelida, and many other phyla of worms,to move about by an hydraulic system driven by fluid pressure. The origin and form ofthese body cavities characterize different phyla. While these phyla are soft-bodied.others are protected and supported by various types of skeletons,such as shells in Mollusca and a jointedexoskeleton in Arthropoda. The divisionof the body into segments allowed forspecialization of parts of the body.In arthropods, this has led tothe development of segmentalappendages that carry outspecific functions, such assensory perception, feedingand locomotion. Details ofearly embryonic developmentdivide many advanced phylainto two lineages, one leadingthrough the Echinodermata tothe Chordata, the phylum to whichvertebrates belong, the other containingthe bulk of animal phyla. While molecular Phylum enidaria, page 520analyses have confirmed many of our ideasabout the course of evolution based on anatomy and development, there are anumber of instances where they are at variance. Hence, our classificatory systemis undergoing revision. Furthermore, the continual identification of new species ofinvertebrates indicates that we are nowhere near their full inventory, and certainlyfar from understanding their vital roles in the sustainability of ecosystems

PhylumChordataInvertebrateChordates

SubphylumUrochordataSeasquirts

SubphylumCephalochordataLancelets

PhylumPoriferaSponges

PhylumCnidariaCnidarians(sea anemones,corals,jellyfishes,etc.)

PhylumPlatyhelminthesflatworms

PhylumNematodaRoundworms

PhylumMolluscaMollusks(bivalves,snails,squids,etc.)

PhylumAnnelidaSegmentedworms

PhylumArthropodaArthropods Phylum Mollusca. page 525

SubphylumCheliceratarf-euceretes

ClassArachnidaArachnids

ClassMerostomataHorseshoecrabs

ClassPycnogonidaSeaspiders

~ClassArachnida, page 536

SubphylumMyriapodaMyriapods(centipedes,etc.)