^ THE DOGFISH. PHYLUM CHORDATA 



passage the pericardio-peritoneal canal, leads from one to the 

 other below the oesophagus, and can be found in dissection by 

 probing with a seeker behind and above the heart. From the 

 peritoneal cavity the two small abdominal pores lead to the 

 exterior, one on either side of the vent. The gape of the mouth 

 is edged with several rows of teeth, which as we have seen, are 

 simply enlarged scales ; their function is only to hold the food, 

 not to cut or chew it. They lie in a part of the skin which passes 

 over the jaw and is tucked into a groove within it and are not 

 in any way attached to the skeleton of the jaw. Teeth formed m 

 the groove are continually being carried forward over the edge 

 of the jaw by the growth of the skin, so that as they wear away 



'^--- hyp. 



Fig. 243. — The hinder wall of a gill pouch of the dogfish. 



br a Branchial arch, flexed because the pharynx floor is raised ; eh., eb'., extrabranchials ; g., giU ; hyp. 

 *' hypobranchials in section ; m.e.o., margin of external opening. 



they are replaced by new ones, a type of dentition called poly- 

 phyodont. The pharynx is only distinguished from the buccal 

 cavity by possessing the inner openings of the spiracle and gill 

 clefts. These are placed between the arches of the visceral skeleton, 

 the first gill cleft lying between the hyoid and first branchial arches. 

 The clefts slant backwards so that the outer opening of each is at 

 some distance behind the inner. The spaces between inner and outer 

 openings, known as gill pouches, are considerably taller than the 

 openings themselves. On each wall of each pouch except the last 

 he a number of folds which constitute a gill or hemibranch 

 (Fig. 243). This is highly vascular, and in fresh specimens has 

 consequently a bright red colour. The last pouch has a gill on 

 the anterior side only. The spiracle is a small cleft of the same 

 series as the gill clefts, and bears on its front side a vestige of a gill, 

 known as a pseudobranch. The bars of body- wall between the 

 mouth and spiracle, between the spiracle and first gill slit, and 



