FINS 



321 



isc.pu- 



ac. 



h. r 



middle line. At about the middle of the posterior surface of each 

 scapulocoracoid is a hollow, the glenoid facet, with which the 

 skeleton of the fin articulates ; this facet marks also the division 

 into ventral coracoid and dorsal scapular regions. The coracoid 

 is broad and flat, and supports the floor of the pericardium ; the 

 scapular part is a curved rod. The pelvic or hip girdle (Fig. 242) is a 

 stout straight bar running transversely just in front of the anus ; 

 it is formed by the fusion of right and left ischiopubic cartilages, 

 each of which has a knob, 

 the iliac process, at its 

 outer end. The fin articu- 

 lates with an acetabular 

 facet on the hinder border 

 at the base of the iliac 

 process. 



The skeleton of the 

 paired fins is similar to that 

 of the unpaired fins, that is, 

 there are basalia, radialia, 

 polygonal plates and a 

 double series of dermo- 

 trichia. The pectoral fin 

 has three basals, called 

 pro-, meso-, and meta- 

 pterygia, of which the first 

 is anterior and smallest, 

 the last posterior and 

 largest ; all these articulate 

 with the girdle, but they 



are not movable on each other, the pro- and mesopterygia have 

 one radial each, the metapterygium has several. In the pelvic 

 fin there is a single basal, the basipterygium, which bears several 

 radials. In the male there is also a long piece of cartilage 

 supporting the clasper. 



Fig. 242. — The skeleton of the pelvic fins 

 and girdle of a female dogfish. 



flf. , Acetabular surface; bp., basipterygium; h.r., horny 

 rays ; //., iliac process ; isc.pu., ischiopubic region ; 

 rad., cartilaginous rays. 



CCELOM AND ALIMENTARY SYSTEM 



The perivisceral cavity (coelom) is divided into two parts, the 

 small pericardial cavity just in front of the pectoral fins, and the 

 large peritoneal cavity behind it, between the two pairs of fins. The 

 two cavities are divided by a membranous septum, but a narrow 



