xii PHYLUM CHORDATA 367 



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 

 situated one in front of each angle of the mouth. A small 

 rounded aperture, the spiracle placed just behind the eye 

 leads into the pharynx. Five pairs of slits running 

 vertically on each side of the neck the branchial slits 

 also lead internally into the pharynx. A large median 

 opening on the ventral surface at the root of the tail, 

 between the pelvic fins, is the vent, or anus, 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 aperture, lead into narrow passages opening into the 

 abdominal cavity. 



The skeleton is composed entirely of cartilage, with, in 

 certain places, depositions of calcareous salts. As in 

 Vertebrates 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. 



The spinal column is distinguishable into two regions the 

 region of the trunk and the region of the tail. In the trunk 

 region each vertebra (see Fig. 195, A, B) consists of a centrum 

 (c.) neural arch (. a.) and transverse processes (tr. //-.). 

 In the caudal region there are no transverse processes, but 

 inferior or haemal arches (C, D, h. a.) take their place. The 

 centra of all the vertebras are deeply biconcave or amphicailous, 

 having deep conical concavities on their anterior and posterior 

 surfaces. Through the series of centra runs the notochord, 

 greatly constricted in the centrum itself, dilated in the large 



