342 



MANUAL OF ELEMENTARY ZOOLOGY 



dogfish is a metazoan animal. It is triploblastic and 

 ccelomate. It has a large, ccelomic perivisceral cavity and 

 a closed circulation. It is bilaterally symmetrical. It is 

 segmented, though the primary segmentation is best seen 

 in the early stages of development and is represented in the 

 adult only by the arrangement of the muscles of the body- 

 wall, the segmentatidn which is found in the backbone 

 and nervous system arising later. It is chordate (p. 339). 



B 



Fig. 246. — Placoid scales. / 



A . A portion of the skin of the rough hound as seen under a hand lens ; B, a single 

 scale removed from the skin ; C, the same in section (diagrammatic). 



&., Base of the scale ; c, the same in section ; d., dentine ; e. t enamel ; 

 /., pulp cavity. 



Lastly, like the frog, it is a backboned or vertebrate animal. 

 This term implies more than the possession of a backbone. 

 The Vertebrata are a large group of animals which have in 

 common, besides the features we have just mentioned, the 

 following: (1) they possess an internal skeleton of bone or 

 cartilage, part of which forms a skull and backbone; 

 (2) their central nervous system, which is hollow and 

 dorsal like that of all chordate animals, consists of a spinal 

 cord and a complicated brain ; (3) the gill clefts, which they 

 all possess during some period of development, are few and 



