XIII 



PHYLUM CHORDATA 



tr. 



ae. 



is composed externally of obliquely running fibres, and represents 

 one of the two obh'qt'.r muscles of the abdomen of higher forms. 

 Mesially this passes into a median band of longitudinally running- 

 fibres corresponding to a primitive rcctus. 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, depress- 

 ors, and adductors. 

 A series of circular 

 muscles pass between 

 the cartilages of the 

 visceral arches, and 

 when they contract 

 have the effect of con- 

 tracting the pharynx 

 and constricting the 

 apertures. A set of 

 muscles pass between 

 the various arches 

 and act so as to ap- 

 proximate them ; and 

 a broad sheet of longi- 

 tudinal fibres divided 

 into myomeres ex- 

 tends 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 



pi the Fish OCCUr FIG. TOO. A Torpedo-Ray with the electric organs dissected 



ill Several Fl-ism out - On the right the surface oiilv of the electric organ (ot) 



rrn is shown - On the left the nerves passing to the organ are 



brancllS. I hey are dissected out. The roof of the skull is removed to bring the 



i * _j -I * i brain into view. l,r. branchife ; /, spiracle; o, eyes; tr. 



developed 111 trigeminal ; tr'. its electric branch ; <. vagus ; /, fore-brain ; 



thp "Flon-H-io "Ro^- n > mid-brain; ///, cerebellum; ir, electric lobe. (From 



Ullfc; ttiU S Gegenbaur.) 



(Torpedo and Hypnf>*) 



(Fig. 790) 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 variou 

 parts of the organ. These are derived mainly from four nerv 



M 2 



