xni PHYLUM CHORDATA 171 



process. A lateral iliac process, which becomes highly developed 

 in the Holocephali, is sometimes represented, and may attain 

 considerable dimensions. The pelvic fin has usually two basal 

 cartilages, representing the pro- and meta-pterygia, but the former 

 is often absent. In the male special cartilages attached to the 

 metapterygia support the claspers. With the basal cartilages 

 of both pectoral and pelvic fins are connected a number of jointed 

 cartilaginous fin-rays supporting the expanse of the fin. 



The arrangement of the muscles is simple. The trunk-muscles 

 are divided into a pair of dorsal and a pair of ventral divisions, 

 each composed of many myomeres with intercalated myocommata 

 (Fig. 768, p. 68), following a metameric arrangement. The ventral 

 part, where it forms the muscles of the wall of the abdominal cavity, 

 is composed externally of obliquely running fibres, and represents 

 one of the two oblique muscles of the abdomen of higher forms. 

 Mesially this passes into a median band of longitudinally running 

 fibres corresponding to a primitive rectus. The muscles of the limbs 

 are distinguishable into two main sets those inserted into the 

 limb-arch and those inserted into the free part of the appendage. 

 The latter, according to their insertion, act as elevators, depressors, 

 or adductors. A series of circular muscles pass between the carti- 

 lages of the visceral arches and, when they contract, have the 

 effect of contracting the pharynx and constricting the apertures. 

 A set of muscles pass between the various arches and act so as to 

 approximate them ; and a broad sheet of longitudinal fibres divided 

 into myomeres extends forwards from the shoulder-girdle to the 

 visceral arches. 



Electric organs organs in which electricity is formed and 

 stored up, to be discharged at the will of the Fish occur in several 

 Elasmobranchs. They are best developed in the Electric Rays 

 (Torpedo and Hypnos, Fig. 848), in which they form a pair of large 

 masses running through the entire thickness of the body, between 

 the head and the margin of the pectoral fin. A network of strands 

 of fibrous tissue forms the support for a number of vertical prisms, 

 each divided by transverse partitions into a large number of com- 

 partments or cells. Numerous nerve-fibres pass to the various 

 parts of the organ. These are derived mainly from four nerves, 

 which originate from an electric lobe of the medulla oblongata, with 

 a branch from the trigeminal. By means of the electric shocks 

 which they are able to administer at will to animals in their 

 immediate neighbourhood, the Torpedo-Rays are able to ward off 

 the attacks of enemies and to kill or paralyse their prey. In the 

 other Rays in which the electric organs are developed they are 

 comparatively small organs situated at the sides of the root of the 

 tail. In all cases the cells are formed from metamorphosed muscular 

 fibres. * 



Luminous organs by the agency of which a phosphorescent 



