368 MANUAL OF ZOOLOGY sect. 



Of the lateral fins there are two pairs, the pectoral and the 

 pelvic. The pectoral are situated at the sides of the body, 

 just behind the head. The pelvic, which are the smaller, are 

 placed on the ventral surface, close together, in front of the 

 middle of the body. In the males the bases of the pelvic fins 

 are united together in the middle line, and each has connected 

 with it a clasper or copulatory organ. The latter is a stiff rod, 

 on the inner and dorsal aspect of which is a groove leading 

 forwards into a pouch-like depression in the base of the fin. 



The mouth — a transverse, somewhat crescentic opening 

 — is situated on the ventral surface of the head, near its 

 anterior end. In front and behind it is bounded by the 

 upper and lower jaws, each bearing several rows of teeth 

 with sharp points directed backwards. The nostrils are sit- 

 uated one in front of each angle of the mouth, with which 

 each is connected by a wide groove, the nasobuccal groove. 

 A small rounded aperture, the spiracle, — placed just behind 

 the eye, — leads into the large mouth-cavity or pharynx. 

 Five pairs of slits running vertically on each side of the neck, 

 the branchial slits, also lead internally into the mouth- 

 cavity. A large median opening on the ventral surface at 

 the root of the tail, between the pelvic fins, is the opening 

 leading into the cloaca, or chamber forming the common 

 outlet for the intestine and the renal and reproductive organs. 

 A pair of small depressions, the abdominal pores, situated 

 behind the cloacal opening, lead into narrow passages open- 

 ing into the abdominal cavity. 



The skeleton is composed entirely of cartilage, with, in 

 certain places, depositions of calcareous salts. As in Verte- 

 brates in general, we distinguish two sets of elements in the 

 skeleton, — the axial set and the appendicular, the former 

 comprising the skull and spinal column, the latter the limbs 

 and their arches. 



