368 Vertehrata. 



The pelyiSj whicli is not connected with the vertebral column 

 in Fishj is^ in the Selachians, an unpaired transverse ventral cartilage. 

 In the Dipnoi, too, it is an unjointed cartilaginous plate. In the 

 Ganoids and Teleosteans it is divided into two halves, closely apposed 

 to one another, and except in the Chondrostei it is partially, or entirely, 

 ossified. The hind limb closely resembles the fore limb in 

 structure ; in the Selachians there are two large basalia provided with 

 radialia ; in the Ganoids the basalia are degenerate or absent ; 

 this is also the case in the Teleostei, in which the skeleton of the hind 

 limb is represented by a few short radialia only, it is even less 

 developed than the fore limb. In the Dipnoi the conditions are 

 the same for both hind and fore limbs. 



The muscular system is distinguished by the feeble condi- 

 tion of the limb muscles, as compared with the powerful development 

 of the trunk and tail musculature, which extends along the whole 

 length of the body in the form of four large muscle plates, two on 

 each side. Each of these is broken up, by transverse septa 

 (myocommata), into a series of short segments, corresponding to the 

 vertebrae in number (myomeres) : in the dorsal muscles, and the 

 caudal portions of the ventral muscles, the septa are bent in a 

 peculiar way. In these muscles there are in many Pish numerous 

 fine rib-like bones, the epipleurals, which are attached at one end 

 to the ribs or to the vertebrce. They are ossifications in the 

 myocommata and serve as supports to the muscles. 



The electric organs present in many Pisces generate electricity, wHcli 

 can be discliarged at will : for instance, wten the animal is caught. The 

 essential elements of these organs are plates, modified muscle fibres, for 

 the entire organ is a modified muscle : it is richly supplied with nerves, and 

 fibres enter each plate and branch freely on one side. The plates are bound 

 together by connective tissue; the organ is frequently composed of closely 

 apposed columns, each of which consists of a number of plates. The electric 

 organs lie in different regions of the body in different Pish; the most powerful 

 apparatus is that of the Electric Skate (IWpetZo), of the Sheath-fish or 

 Thunderer [Malapterurus), and of the Electric Eel {Gymnotus) ; but 

 feebler organs also occur in some other forms, e.g., in our native Skates {Baja), 

 where they are elongate spindles lying one on either side of the tail. 



The brain of Pisces is of small size, and does not fill the 

 cranial cavity, which is chiefly occupied by the dura mater, a 

 membrane of considerable thickness, consisting chiefly of adipose 

 tissue. The olfactory lobes are usually large, and often of 

 considerable length, since the olfactory organ lies anteriorly, far 

 away from the brain. In many Teleostei, the fore brain is very 

 small, smaller than the mid brain; the hind brain, on the other 

 hand, is usually very well developed. 



The olfactory organs generally occur at the anterior end 

 of the head, as a pair of pits whose mucous membrane is usually in 

 radial folds. The opening of each pit is single in many forms (some 



