496 MANUAL OF ZOOLOGY. 



of the gills is the same as the above, there is only a single 

 branchial aperture or gill-slit externally^ and this is protected 

 by a rudimentary operculum and branchiostegal rays. 



III. Another character in the Elasmobranchii, shared, how- 

 ever, by many of the Ganoids, is the structure of the intestinal 



Fig. 268. A, Head of Piked Dog-fish (Spinax\ showing the transverse mouth on 

 the under surface of the head, and the apertures of the gill-pouches. B, Diagram 

 of the structure of the gill-pouches : o o External apertures ; i i Apertures leading 

 into the pharynx ; J s Gill-sacs, containing the fixed gills. 



canal. The intestine is extremely short; but, to compensate 

 for this, there is a peculiar folding of the mucous membrane, 

 constituting what is known as the " spiral valve." The mucous 

 membrane, namely, from the pylorus to the anal aperture, is 

 folded into a spiral reduplication, which winds in close coils 

 round the intestine, like the turns of a screw. By this means 

 the absorptive surface of the intestine is enormously increased, 

 and its shortness is thus compensated for. 



It is to be noted that some high authorities are in favour 

 of the view that the Elasmobranchii are to be regarded as a 

 distinct class, and not as merely an order of the class Pisces. 



The order Elasmobranchii is divided into two sub-orders : 

 the Holocephali, characterised by the mouth being terminal in 

 position, and there being only a single gill-slit ; and the Plagio- 

 stomi, in which the mouth is transverse, and placed on the 

 under surface of the head (fig. 268, A), and there are several 

 branchial apertures on each side of the neck. 



SUB-ORDER A. HOLOCEPHALI. This sub-order includes 

 certain curious fishes, of which the only living forms are the 

 Chimaridce. The notochord is persistent : but the neural arches 

 and transverse processes are cartilaginous. The jaws aje 

 bony, and are covered by broad plates representing the teeth. 

 The exoskeleton consists of placoid granules. The first ray 

 of the anterior dorsal fin is in the form of a powerful defensive 

 spine, like the " ichthyodorulites " of many fossil fishes. The 

 ventral fins are abdominal, and the tail is heterocercal. There 

 is only a single external gill-aperture, covered with a gill-cover 

 and branchiostegal membrane ; but only a small portion of 



