276 



PRINCIPLES OF ANIMAL BIOLOGY 



Brachiopoda. Marine tentaculate animals with a calcareous shell, composed of 

 two unequal valves, a dorsal and a ventral. (Fig. 245.) 

 Gephyrea. Wormlike animals of doubtful affinities. 



Fig. 245. — A brachiopod. Left, the shell; right, the animal. 



Phylum 10. Chordata. — The animals of this phykim have at some 

 stage a skeletal axis called the notochord, gill slits in the embryo or adult, 

 and a nerve cord dorsal to the alimentary canal. (In preceding phyla 

 when a nerve cord is present it is ventral to the alimentary tract.) This 



Fig. 246. — Balanoglossus. {From Carolina 

 Biological Supply Co.) 



Fig. 247. — A tunicate. {From Carolina 

 Biological Supply Co.) 



phylum includes a number of degenerate animals such as Balanoglossus 

 (Fig. 246) and the tunicates (Fig. 247) which must be included here 

 because of the presence of the notochord in the embryo. It also includes 

 the amphioxus (Fig. 248), a fishlike animal in which the notochord is the 



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Fig. 248. — Amphioxus. 



Fig. 249.^ — Lamprey. {From Carolina 

 Biological Supply Co.) 



permanent skeletal axis. The remaining chordates are called vertebrates 

 from the fact that the notochord becomes invested with cartilage which 

 is segmented to form a vertebral column. In some lower forms the carti- 

 laginous vertebrae and the notochord which they surround persist 

 throughout the life of the animal, but in the higher forms the cartilage is 

 replaced by bone and the notochord disappears. 



Fig. 250. — Hagfish. {From Carolina Bio- 

 logical Supply Co.) 



Fig. 25 L — A shark. {From Carolina Bio- 

 logical Supply Co.) 



At the bottom of the vertebrate scale are the lampreys (Fig. 249) and 

 hagfishes (Fig. 250) which are eellike in form but have no jaws and no 

 lateral fins. The skeleton is made of cartilage only. Some of the lam- 

 preys inhabit fresh water and lay their eggs in nests made by pulling up 

 stones from the bottoms of brooks. Next above these in the scale are 

 the sharks (Fig. 251), skates (Fig. 252), and rays, whose skeletons are also 

 cartilaginous but which have jaws. Their skin is armored with a type of 

 scale having a tooth or spine mounted on a flat base. The dried and proc- 



