ELASMOBRANCHII. 371 



ries a series of branchial laminae attached to each side like the 

 leaves of a book. By means of these septa a series of branchial 

 sacs or pouches are formed, each of which opens internally 

 into the pharynx by a separate slit, and communicates exter- 

 nally with the water by a separate aperture placed on the side 

 of the neck (fig. 141, B). The arrangement of the gills being 

 such, there is, of course, no gill-cover, and no branchiostegal 

 membrane or rays. In one section of the order, however 

 viz., the Holocephali though the internal structure of the gills 

 is the same as the above, there is only a single branchial aper- 

 ture or gill-slit externally, and this is protected by a rudiment- 

 ary operculum and branchiostegal rays. 



Fig. 141. 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 j Gill-sacs, containing the fixed gills. 



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

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

 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. 



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. 141, A), and there are several 

 branchial apertures on each side of the neck. 



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

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

 Chimceridce. The notochord is persistent ; but the neural 

 arches and transverse processes are cartilaginous. The jaws 

 are bony, and are covered by broad plates representing the 



