42O MANUAL OF ZOOLOGY. 



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 communi- 

 cates externally with the water by a separate aperture placed 

 on the side of the neck (fig. 178, 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. 



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. 178, 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 

 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 hetero- 

 cercal. There is only a single external gill-aperture, covered 

 with a gill-cover and branchiostegal membrane ; but only a 

 small portion of the borders of the branchial laminae is free. 

 The mouth is placed at the extremity of the head. 



The best-known living representative of the sub-order is the 

 Chinuzra monstrosa (fig. 179, B), commonly known as the 

 " king of the Herrings." In Chimara there is only one ap- 

 parent gill-slit, but the gills really adhere to the integument by 



