SCYLLIUM 51 



that divides the myotomes horizontally, and are called " dorsal " 

 ribs (see p. 307). 



Fins. — The median fins are supported by jointed cartila- 

 ginous rods or radials. These were originally direct con- 

 tinuations of the neural and haemal arches, but as a result of 

 the shortening of the bases of the fins (or concentration), the 

 radials no longer correspond with the vertebras, except in 

 the ventral lobe of the tail. In addition to the cartilaginous 

 radials, the web of the fin is supported by horny dermal fin- 

 rays, close under the skin on each side of the radials. These 

 rays, or ceratotrichia, are more numerous than the radials. 



The paired fins also have an internal skeleton of cartila- 

 ginous radials, and are anchored to the body by girdles lying 

 in the body- wall. The pectoral girdle is a half-hoop of cartilage 

 set transversely to the long axis of the body, with the free ends 

 pointing upwards. On each side is a hollow, the glenoid 

 cavity, into which the cartilages of the fins fit. The latter 

 cartilages are the most proximal radials, which form three 

 large cartilages, the pro-, meso-, and metapterygia. The 

 ventral portion of the pectoral girdle is termed the coracoid 

 region ; from the glenoid cavity to the free tips which project 

 dorsally, the cartilage is known as the scapular region. 



The pelvic girdle is formed by a transverse cartilage, at 

 each end of which an elongated backwardly-directed basi- 

 pterygium is articulated. This basipterygium forms the axis 

 of the pelvic fin, and bears a number of cartilaginous radials 

 on its anterior border. 



The pectoral and pelvic fins, as well as the median fins, 

 have their webs supported by horny dermal fin-rays, the 

 ceratotrichia. 



Alimentary System. — The mouth leads into the pharynx, 

 the sides of which are pierced by the spiracle and the gill-slits. 

 The food consists of fair-sized pieces of prey, seized by the 

 jaws, and in no danger of being lost through the gill-slits. 

 Behind the pharynx is the oesophagus which leads into a large 

 stomach. In its formation, the gut has been kinked to the left, 

 so that the stomach is a well-defined region. Ventral to it is a 

 large liver with a gall-bladder from which a bile-duct leads 



