COLD-BLOODED VERTEBRATA 



377 



(Petromyzon) is an example of these. It has an interesting 



larva, the Ammocoetes, which feeds precisely as the lancelet 



does, the thyroid gland opening on the floor of the pharynx 



and, like the endostyle, secreting mucus, which is carried 



upwards by peripharyngeal bands and backwards along the 



roof of the pharynx to the gullet. 



Among fishes, three sub-classes may be recognised — the 



Cartilaginous 



Fishes: i. fias-Fi s hes or Elasmo- 

 mobranchii : . . .. , _ 



The Skate. branchii, the Bony 

 Fishes or Teleo- 

 stomi, and the Lung Fishes or 

 Dipnoi. The Elasmobranchii 

 have no bones and no air- 

 bladder, their gill clefts are 

 uncovered, and they wear 

 placoid scales (p. 343). Be- 

 sides the dogfishes and sharks, 

 skates and rays belong to this 

 group. In a skate, the body 

 is transformed by an immense 

 development of the pectoral 



ks. p.c.s. 



au. t auricle ; c.a., conus arteri- 

 osus ; c./i.s., common hepatic 

 sinus; d.C, ductus Cuvieri ; 

 k.d.i hepatic sinuses ; p.c.s., 

 posterior cardinal sinus ; v., 

 ventricle ; v.ao., ventral aorta. 



fins, which stretch forward to FlG . 269. — A semi-diagram- 

 the end of the snout and matic view of the heart 



spread outwards, SO that the and neighbouring vessels 



width of the fish is much of a skate. 



greater than its depth, and by a - i - a -< Afferent branchial arteries 

 . r , x ., ' a.c.s. t anterior cardinal sinus; 



a narrowing of the tail to a 

 whip-like organ. The skate 

 lives upon the ground, draw- 

 ing its breaths of water through 

 the large spiracles, which are 

 upon the top of the head, whereas the gill clefts are 

 on the smooth, white under side with the mouth. The 

 spiracle is used for taking in water in the dogfish also, 

 but is there not of the same importance, since the fish, 

 not being flattened upon the ground, can generally obtain 

 most of its water through the mouth. The arrangement 

 of the principal blood-vessels of the Common Skate, 

 which differs a little from that of the dogfish, is shown in 

 Fig. 269. In the nervous system (Fig. 270) there will be 



