148 FISHES CHAP. 
Invariably present. The olfactory organs are obviously paired, and 
they are distinct from the hypophysis. Paired limbs are present. 
As previously stated, the true Fishes form the second of the 
six “classes” into which the Craniata are divided. As compared 
with the higher Craniata, their distinctive characters may be 
concisely stated as follows :— 
Fresh water or marine Gnathostomata, which in their shape 
and in method of breathing are adapted for an aquatic life. 
Throughout life their respiratory organs are in the form of 
vascular processes (gills) derived from the walls of the branchial 
clefts, and supported by a series of branchial arches. The 
principal organ of locomotion is the powerful muscular tail; in 
addition, however, there are paired fins, pectoral and pelvic, 
corresponding to the fore- and hind-lmbs of the terrestrial 
Craniata, and possessing a supporting cartilaginous or bony 
skeleton (“ichthyopterygium”) which cannot readily be com- 
pared with the limb-skeleton of the latter. Fishes also possess 
a system of median fins, supported by a special skeleton of their 
own. An exoskeleton of dermal spines or denticles, scales or bony 
plates, is usually present. Except in one group, the Dipnoi, the 
heart has but one auricle, and receives only venous blood, which 
it forces, first, through the blood-vessels of the gills, and thence, 
as arterial blood, through the vessels of the body generally. An 
air-bladder is frequently present, and serves as a hydrostatic organ 
or float, but in a few cases it may act as a lung, and helps the 
gills in the work of respiration. The paired olfactory organs rarely 
communicate with the oral cavity by internal nostrils. Peculiar 
cutaneous sense-organs are disposed in linear tracts along the sides 
of the body (lateral line sensory organs), and on the head, and 
appear to be specially associated with a life in water. 
Fishes may be divided into the following “sub-classes,” and 
these in turn may be subdivided into various “orders” and 
“ sub-orders ” :— 
(i.) ELASMOBRANCHII ; e.g. Sharks, Dog-Fishes, Skates, and Rays. 
(1) Pleuropterygii tT; eg. Cladoselache. 
(2) Ichthyotomi t ; eg. Plewracanthus. 
(3) Acanthodei7 ; eg. Acanthodes. 
(4) Plagiostomi. 
(a) Selachii ; eg. many extinct and all living Sharks 
and Dog-Fishes. 
(b) Batoidei; eg. Skates and Rays. 
(5) Holocephali; eg. Chimaera and Callorhynchus. 
