PISCES. 437 



There is an extensive system of lateral line sense organs 

 innervated chiefly by the Vth, VJIth, and Xth cranial 

 nerves. All fishes have a well-developed vertebral column 

 and visceral arches ; the first arch is always modified into 

 upper and lowxr jaw^s. 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. — Teleostomi, 



The Teleostomi 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 Teleostomi 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 oesophagus 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 freshw^ater forms 

 and some marine they are demersal or deposited on the 

 bottom : they are small and numerous. 



The Teleostomi 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 

 Elasmobranchii^ whilst the condition of the urinary organs 

 and the brain are at a decidedly lower level. 



