The Classification of Animals. 203 



Sub-class 2. Pisces.— The fishes, as is the case with all the 

 remaining Vertebrates, have two pairs of limbs. These limbs 

 are, however, what is called an Ichthyopterygium ; they are 

 not formed on the plan of the five-fingered hand, or foot, of 

 the higher Vetebrates, but are made up of a larger or smaller 

 number of cartilaginous, or bony rays, which cannot be securely 

 reduced to the type of the cheiropterygium. 



This is really the only distinguishing feature which absolutely 

 separates all fishes from all Amphibians. The bulk oif fishes, 

 however, are so characterized by the fact that they breathe by 

 gills only, and, if there are simple lungs, by gills also : by the 

 fish-like form, with its unpaired fins, and the great develop- 

 ment of the lateral line. The Pisces are divisible into the 

 four following groups : — 



1. Elasmobranchii. — The sharks and skates. The skin is 

 smooth, with scattered spines forming, in some cases, a close 

 investment (shagreen). The skeleton is cartilaginous, the 

 notochord largely persistent. The swim-bladder is entirely 

 wanting ; the intestine, furnished with an extensive spiral valve, 

 opens into a cloaca. 



2. Holocefhali. — This limited group of fishes contains the 

 chimsera of northern seas and some other forms. The skeleton 

 is cartilaginous, as in sharks ; but the mandible articulates 

 directly with the mass of cartilage forming the skull, there being 

 no free palatoquadrate or hyoid suspensorium. There is a 

 dermal flap (operculum) covering the gill-slits, which are thus 

 not exposed as in sharks. The spiral valve is less conspicuous, 

 and the anus is separate from the urogenital pore. There is no 

 swim-bladder. 



3. Teleostomi. — This group includes the Ganoids and Tele- 

 osteans. Examples of the former are the sturgeon, Polypterv,s 

 of the Nile, etc; of the latter, the vast majority of the 

 familiar fishes, pike, perch, sole, herring, etc. The skeleton is 

 sometimes largely, but never entirely, cartilaginous; in the 

 Teleosts it is nearly always greatly ossified. The body is 

 generally invested with bony plates or scales. The intestine 

 may, or may not, have a spiral valve, and be furnished, or not 

 furnished, with pyloric caeca. There is no cloaca ; the gill-slits 



