ELASMOBRANCHII. 395 



rially from those of the Bony and Ganoid fishes. In the case 

 of the Sharks, and Rays, the structure of the gills is as follows : 

 — The branchial arches are fixed, and the branchial laminx 

 are not only attached by their bases to the branchial arches, 

 but are also fixed by the whole of one margin to a series of 

 partitions, which divide the branchial chamber into a number 

 of distinct pouches (fig. 149). Each partition, therefore, car- 

 ries a series of branchial laminas 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 sHt, and communi- 

 cates externally with the water by a separate aperture placed 

 on the side of the neck (fig. 149, B). The arrangement of the 

 gills being such, there is, of course, no gill-cover, and no bran- 

 chiostegal 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 aperture or gill-slit externally, and this is protected 

 by a rudimentary operculum and branchiostegal rays. 



Fig. 149 — A, Head of Piked Dog-fish {Spinax), showing the transverse mouth or» 

 the under surface of the head, and the apertures of the gill-pouches. 6, Diagram 

 of the structure of the giU-pouches : o o External apertures; ii Apertures leading 

 into the pharynx ; s s 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 



