PISCES. 437 
There is an extensive system of lateral line sense organs 
innervated chiefly by the Vth, VlIth, and Xth cranial 
nerves. All fishes have a well-developed vertebral column 
and visceral arches; the first arch is always modified into 
upper and lower jaws. As in Amphioxus and the Cyclo- 
stomata, the greater portions of the body and the longi- 
tudinal muscles form the organs of locomotion. In the 
great majority of fishes the heart is two-chambered and 
respiratory, and the posterior venous system consists of 
paired cardinal veins or sinuses. 
Development is mainly embryonic, the egg containing 
much yolk, but a free larval form is found in many. 
ORDER I.—TZeleostomt. 
The Zeleostomi are'called the “bony fishes” because 
their skeleton is almost entirely formed of bone. Hence 
a cod’s skull is a very different structure from that of a 
skate, for it consists of a great number of bones which, 
to a large extent, fall apart when the connective tissue is 
destroyed by boiling. 
The Zeleostomi usually have cycloid, ganoid or ctenoid 
scales, but some have none. The tail is nearly always 
homocercal. The genital organs communicate with the 
exterior by genital ducts in both sexes, and the genital 
and anal apertures are separate, hence there is no cloaca. 
The gills are enveloped in a bony operculum. In con- 
nection with the cesophagus many have a large air-bladder 
lying under the vertebral column. In the brain the cere- 
brum is very small; the optic lobes and the cerebellum are 
large. The kidneys are purely mesonephric. Development 
is embryonic in its earlier stages, but the young are hatched 
as larvae with a large dependent yolk-sac. The eggs in 
many marine types are pelagic, but in the freshwater forms 
and some marine they are demersal or deposited on the 
bottom : they are small and numerous. 
The Zeleostomi show a peculiar combination of structural 
characters, some, such as the ossification of the skeleton 
and the absence of a cloaca, placing them above the 
Elasmobranchit, whilst the condition of the urinary organs 
and the brain are at a decidedly lower level. 
