164 ZOOLOGY 



SECT. 



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. 



Digestive System. Teeth are developed on the palato- 



quadrate and on Meckel's cartilage. They are arranged in several 



parallel rows, and are developed from a groove at the back of the 



jaw, successive i;ows coming to the front, and, as they become 



worn out, falling off and becoming replaced by others. In the 



Sharks the teeth are usually large and may be long, narrow, and 



pointed, or triangular with serrated edges, or made up of several 



sharp "cusps ; in the Rays, however, the teeth are more or less 



obtuse, sometimes, as in the Eagle-Rays, forming a continuous 



pavement of smooth plates covered with enamel, adapted to 



crushing food consisting of such objects as Shell-fish and the like. 



The Sharks have a prominent tongue supported by the median basi- 



hyal ; this is entirely or almost entirely absent in the Rays. There 



are no salivary glands. The various divisions of the enteric canal are 



similar in all the members of the class to what has already been 



described in the case of the Dog-fish. A spiral valve is always present 



in the large intestine, though its arrangement varies considerably in 



the different families. In some cases (e.g. Oarcharias) the fold is 



not a spiral one, but, attached by one edge in a nearly longitudinal 



line to the intestinal wall, is rolled up in the shape of a scroll. 



A ccecum occurs in Laemargus. The rectum always terminates in 



a cloaca, into which the urinary and genital ducts also lead. There 



is always a voluminous liver and a well-developed pancreas. 



A thyroid lies in the middle line behind the lower jaw. A 

 representative of the thymus lies on either side, a little below 

 the upper angles of the branchial clefts. 



The respiratory organs of the Elasmobranchii always have 

 - the general structure and arrangement already described in the 

 case of the Dog-fish. 



In addition to the gills supported on the hyoid and branchial 

 arches there is also in the Notidanidse a gill in the spiracular cleft 

 the spiracular gill represented in many others by a rete mirabile 

 or network of blood-vessels. In Selache (the Basking Shark) 

 there are a series of slender rods, the gill-rakers, which impede 

 the passage outwards through the branchial clefts of the small 

 animals on which those Sharks feed. 



Blood System. The heart has in all essential respects the 

 same structure throughout the group. The conus arteriosus is 



