208 



AN INTRODUCTION TO ZOOLOGY 



FIG. 66. The rough 

 hound from Bor- 

 rodaile. 



Note mouth, eye, spi- 

 racle, lateral line, gill 

 clefts, pectoral and pelvic 

 fins, dorsal fins, caudal 

 fin, vertical fins between 

 caudal and pelvic fins. 



cf., upper lobe of caudal 

 fin : cf 1 ., lowerlobe of the 

 same; pl.f., pelvic fin. 



on to the ventral 



in a number of living Amphibia it bears exo- 

 skeletal structures in the form of scales, feathers 

 or hairs. 



External Features and Integument. 



The dogfish is bilaterally symmetrical 

 with a greatly elongated spindle-shaped body 

 admirably adapted for swimming, and it may 

 reach a length of about two feet. 5. catulus 

 is much larger, and may grow to almost double 

 this size. The body is divisible into three 

 regions : the head extends back to the beginning 

 of the pectoral fin and contains the skull, sense 

 organs, buccal cavity and pharynx ; the trunk 

 stretches from that point to the cloaca, and 

 contains the main circulatory organs, the viscera 

 and the ccelom which is confined to this region ; 

 lastly, the remaining part, which occupies 

 slightly more than half the total length, is the 

 tail forming a very efficient organ of pro- 

 gression. 



The fins form a very striking series of thin 

 flat expansions supported by skeletal elements, 

 and are divisible into two groups, the median 

 or unpaired fins, and the lateral or paired fins. 

 Of the four median fins two are placed in the 

 mid-dorsal line, the foremost and larger, or 

 anterior dorsal fin, is about half-way back, and 

 the posterior dorsal fin about two-thirds of the 

 way back. They are triangular in shape, and 

 the anterior of them in the sharks projects 

 from the water when the animal is basking just 

 beneath the surface, and so constitutes for 

 sailors in warmer seas a significant indication 

 of the presence of these voracious fish. The 

 ventral fin is a similar but blunter fin in the 

 mid-ventral line situated at a distance inter- 

 mediate between the two dorsal fins from the 

 front of the animal. The fourth of the unpaired 

 fins, and by far the largest, is the caudal fin, It 

 commences about three-quarters of the way 

 back as a low ridge, and increasing in height 

 passes right round the posterior end of the tail 

 surface. It will be seen on closer inspection 



