664 - Heredity and Evolution 



Class 4. The Ophiuroidea (the Brittle Stars, 

 etc.). Body, small, round, distinctly demar- 

 cated; arms, five, slender, jointed, and ilexi- 

 ble, used for active locomotion; tube feet, 

 in two rows, without suckers, mainly sensory 

 in function; no anus; Mississippian period 

 to present time (Fig. 32-33). 



Class 5. The Crinoidea (the Sea Lilies). Body, 

 cup-shaped, called a calyx, attached to sub- 

 stratum, sometimes by an elongate stalk; five 

 double arms, with numerous side branches, 

 giving a fernlike appearance to the organism; 

 tube leet, suckerless; no madreporite; spines, 

 absent; Cambrian period to present time 

 (Fig. 32-33). 



Relationship to Other Phyla. Even the 

 oldest echinoderm fossils, found in early 

 Cambrian strata, clearly show most of the 

 unique characteristics of the phylum. Con- 

 sequently it is not possible to say precisely 

 when these animals became separated as a 

 distinct group. Echinoderm and chordate 

 animals display several common features, 

 particularly with reference to the embryonic 

 (anil larval) stages. In fact, many zoologists 

 believe that Echinodermata and Chordata 

 both evolved from the same stock (Fig. 29-1 1). 



THE CHORDATA 



Overwhelmingly this last great phylum of 

 the Animal Kingdom is dominated by the 

 Vertebrata. These "back-boned animals" in- 

 clude: all fish, amphibians, reptiles, birds, 

 and mammals — plus some other less familiar 

 forms. Despite their abundance and diversity, 

 however, the Vertebrata are not ranked as 

 an independent phylum. Rather they consti- 

 tute the subphylum Vertebrata in the phylum 

 Chordata. But the other chordate groups are 

 represented by only a lew relatively obscure 

 and unfamiliar animals, namely, the acorn 

 worms, tunicates, and lancelets (see below). 



Main Characteristics of the Phylum Chor- 

 data. As was staled previously (p. 281), chor- 

 date animals display three unique features. 



1. All chordates develop a notochord. This 

 nonsegmented flexible, rodlike structure lies 



embedded in the body wall along the dorsal 

 midline, and serves as a functional encto- 

 skelelon in lower Chordata (Fig. 15-10). But 

 in the higher Chordata (Vertebrata), the 

 notochord appears only transiently in the 

 embryo. It is replaced more or less com- 

 pletely in the adult by a permanent seg- 

 mented axial skeleton, the vertebral column 

 ("backbone"). 



2. All chordates possess a dorsally placed 

 tubular nerve cord (Fig. 15-10). This is the 

 central nervous system. Usually it exhibits 

 an anterior swollen part, the brain, and a 

 slenderer posterior part, the spinal cord. 



3. All chordates have pharyngeal gill chan- 

 nels (Fig. 15-10). Among lower Chordata (up 

 through the fishes) these contain functional 

 gills; but among higher forms (amphibians, 

 reptiles, birds, and mammals) the gill slits 

 are transient embryonic structures that be- 

 come highly mollified in the adult animal. 



Other Characteristics. Like other higher 

 forms, chordates are triploblastic, bilaterally 

 symmetrical, segmented animals, with a com- 

 plete tubular digestive tract and an extensive 

 well-defined coelomic cavity. Typically they 

 have a well-developed closed circulatory sys- 

 tem. The heart occupies an anterior ventral 

 position, whereas the main distributing ves- 

 sel lies dorsally anil conducts the blood in a 

 posterior direction. Generally speaking, the 

 respiratory, excretory, and reproductive or- 

 gans are highly developed and these systems 

 exhibit many homologies, at least through- 

 out all Vertebrata. 



The Lower Chordata: Chordates without 

 "Backbones." In addition to the Vertebrata, 

 there are three other subphvla in the phy- 

 lum Chordata. These are: (1) the Hemichor- 

 data, (2) the Urochordata, and (3) the Cepha- 

 lochordata. All these lower groups are small 

 and all consist entirely of marine species pos- 

 sessing no jaws and no paired appendages. 

 Nevertheless, the lower chordates are very 

 interesting. The)' provide almost the only 

 ilues to the difficult problem of how verte- 

 bate animals arose. The later fossil record 

 is quite plain in regard to how the different 



