ORDERS OF FISHES. 245 
world by a series of apertures placed on the side of the neck 
(fig. 131). The intestine is very short, but to compensate for . 
this, the mucous membrane is thrown into a fold, which winds 
round the intestine in close turns from the pyloric orifice of 
the stomach to the anus, and which thus greatly increases its 
absorbing surface. 
The best-known members of this order are the Sharks and 
Rays, but numerous extinct forms testify to its great abundance 
in past geological epochs. 
Fig. 131.—A, Head of a Piked Dog-fish (Szzax), showing the transverse mouth 
on the under side of the head, and the gill-slits on the side of the neck. B, 
Diagram to illustrate the gill-sacs of the Sharks: 0 0, External apertures ; Z 2, 
Apertures leading into the pharynx; ss, Gill-sacs, containing the fixed gills. 
TABULAR VIEW OF THE DIVISIONS OF THE ELASMOBRANCHII. 
SUB-ORDER I, HOLOCEPHALI.—The mouth placed at the end of 
the head, and the external opening of the gills in the form of a single 
gill-slit. 
The best-known member of this sub-order is the Chimera monstrosa, 
sometimes called the “King of the Herrings.” 
SUB-ORDER II, PLaciostom1.—Mouth transverse, placed on the 
under surface of the head; external opening of the gills in the form of 
several slits on each side of the neck, not protected by a gill-cover. 
fam. a. Cestraphori.—Ex. Port-Jackson Shark. 
fam. b. Selachit,—Ex. Sharks and Dog-fishes. 
fam. c. Batides,—Ex, Rays. 
ORDER VI. Dipnot (Gr. dz, double ; zoé, breath).—The best- 
known, and until lately the only, forms included in this order are 
the very singular Mud-fishes (Lepidostven), which are of great 
interest from the many points of affinity which they exhibit to 
the Amphibia. The body of the mud-fish (fig. 132) is completely 
fish-like, and is protected by a covering of small, horny, over- 
lapping scales, which have the cycloid characters. There are 
