THE ELASMOBRANCH FISHES 207 



A. The common dogfish, Scy Ilium canicula, seen from the right side. The pectoral 



girdle and the wall of the abdominal cavity have been cut away to expose the 

 abdominal viscera, ama, anterior mesenteric artery ; ca, cceliac artery ; cf, caudal 

 fin ; dfi and dfz, the first and second dorsal fins ; gb, gall-bladder ; gs, the five 

 gill-slits ; int, intestine ; h>, intestinal vein ; /, left lobe of the liver, the right 

 lobe is cut away; /ga, lieno-gastric artery (the reference line should point to the 

 hinder of the two arteries) ; //, lateral line ; pc, pancreas ; pgv, posterior gastric 

 vein ; pf, pelvic fin ; pma, posterior mesenteric artery ; r, rectum ; rg, rectal gland ; 

 spi, spiracle ; sj>l, spleen ; sp.v> splenic vein ; st, stomach ; i>f, ventral fin. 



B. Diagram showing the arrangement of the principal venous sinuses of the dogfish. 



acs, anterior cardinal sinus ; cv, caudal vein ; dc, ductus Cuvieri ; /is, hepatic sinus ; 

 ///, heart; j's, jugular sinus; //, right lobe of liver, the left lobe is cut away; /s, 

 lateral sinus ; mtn, metanephros ; ", oesophagus (cut short) ; 0s, orbital sinus; pcs^ 

 posterior cardinal sinus ; pg, pectoral girdle ; rv, renal veins ; sc.v, subclavian 

 sinus ; sv, sinus venosus. 



The paired fins resemble the median fins in that the greater 

 part of their free expansions are supported by numerous fine 

 horny rays, but their bases are very muscular, and supported 

 by a cartilaginous skeleton. 



Immediately in front of and above each pectoral fin is a 

 row of five vertical slits, leading obliquely forwards into as 

 many separate branchial chambers, which in turn communicate 

 by separate apertures with the gullet. On looking into one 

 of these gill-slits the tufts of red highly vascular gill-filaments 

 may easily be seen. Immediately behind each eye is a much 

 smaller orifice communicating with the gullet, but only pro- 

 vided with a rudimentary tuft of gill-filaments. These smaller 

 openings are known as the spiracles. They are really rudi- 

 mentary gill-slits, and are specially large and conspicuous in 

 rays, but are absent in many sharks. The presence of a 

 number of gill-slits opening separately to the exterior, and 

 destitute of any plate-like covering or operculum, is character- 

 istic of though not peculiar to Elasmobranch fishes. The 

 lampreys also have separate gill-openings. Though there are 

 usually five gill-slits (in addition to the spiracle when present) 

 in sharks, dogfishes, and rays, there are some forms in which 

 this number is exceeded. The genus Heptanchus has seven 

 gill-slits, Chlamydoselache and Hexanchus have six. 



A close inspection of the head and snout of the dogfish 

 reveals the presence of a number of minute pores arranged in 

 symmetrical rows. These are the apertures of the mucous 

 canals, peculiar gelatinous-looking tubes filled with mucus, 

 and richly supplied with nerve endings. 



The back and sides of Scy Ilium canicula are of a warm 

 brownish-grey colour, covered with brown spots. The belly 

 and throat are nearly white, and they and the under sides of 



