SHARKS AND HAYS. 
143 
which, in short, are allied to the Batrachians, and through 
them with the reptiles, birds, and mammals. All the fishes 
agree in having a true skull, to which is attached a mova- 
ble lower jaw. The brain is well developed and the blood 
is red. Fishes breathe by gills, which form four arches on 
Dorsal fin. 
Caudal. 
Anal. Ventral. Pectoral. 
Fig. 184.— The Mud-Minnow, with the names of the fins. 
each side of the throat. The body is usually scaled. They 
are mostly oviparous; some bring forth their young alive. 
Sub-Classes of Fishes. 
1. Skeleton cartilaginous; 5-7 pairs of 
gill-openings Elasmohranchii: Sharks, Rays. 
2. Skeleton cartilaginous or bony; 
scales often square, an fimeWedi .. Oanoidei: Sturgeon, Garpike. 
3. Skeleton bony, of numerous sep- 
arate bones; 4 pairs of gills Teleostei: Cod, Perch, etc. 
Sub-Class I. — Elasmobra^^^chii {Selacliians, or Sharks 
arid Rays). 
These are called Elasmobranchs from the strap-like 
gill-openings {elasma, strap, and hrancliia, gill). The 
sharks, though fish-like, are very diflPerent from ordi- 
nary bony fish. Their skeleton or skull is so soft that it 
can be cut with a knife, while the tail is one-sided, the ver- 
tebral column ending in the larger upper lobe. They also 
have from five to seven gill-openiiigs or slits, whereas the cod 
or perch has but one. The skin is either smooth, or with 
miiuite scales, forming shagreen. Both jaws are armed 
^itb numerous sharp^ flattened teeth^ arranged in rows and 
