498 CHORDATA. 



are lost. The function of the fins in swimming and in balancing makes 

 it necessary that they be broad and well-supported plates. Hence numer- 

 ous skeletal parts are present; besides those preformed in cartilage, numer- 

 ous horny or bony rays; further, that all parts should be similar and closely, 

 even if flexibly, bound to each other. Joints are unnecessary except at the 

 base where the fins join the supports and move upon the body. The 

 supports of the paired fins are the girdles, pectoral and pelvic, arched 

 skeletal parts, which in the sharks are held only by muscles, a statement 

 which is true for the pelvic girdle of all fishes. This is why the ventral 

 fins so readily change their place. Their primitive position is at the 

 hinder end of the body cavity (Pisces abdominales, fig. 559). From 

 this point they can move forward to beneath the pectorals (Pisces thoracici, 

 fig. 560), or may even come to lie in front of them (Pisces jugulares) in 

 the throat region (fig. 561). The pectoral arch is united to the verte- 

 bral column in the skates; in the teleosts it is covered by a large mem- 

 brane bone (clavicle or deithrirm), and connected to the epiotic region of 

 the skull by a chain of small bones. 



The dorsal and anal fins are supported by elements, preformed in 

 cartilage, which rest upon the neural or ha?mal spines and in turn support 

 the fin rays. In the caudal fin the rays rest directly upon the spinous 

 processes. Three types of caudal fin are recognized (fig. 10) distinctions 

 of great importance. The primitive type is the diphy cereal, in which 

 the vertebral column extends directly into the middle of the fin, dividing 

 it into symmetrical halves. In the hctcrocercal type the vertebral axis 

 binds upwards at the base of the fin, so that the dorsal part is reduced, 

 the ventral greatly enlarged, the result being extremely asymmetrical, 

 as seen from the exterior. The homocercal fin is symmetrical externally, 

 but in reality is extremely asymmetrical. The end of the vertebral 

 column is bent abruptly upwards, and hence the fin is almost entirely 

 formed of the ventral portion, which is usually divided by a terminal notch 

 into upper and lower halves. The homocercal fin begins with a diphy- 

 cercal and passes through a heterocercal stage in development. 



Tn correspondence with the simple motions the musculature is simple and 

 consists largely of longitudinal muscles divided into myotomes, which are conical 

 \vith the apex in front, and are so inserted in each other that a cross-section gives 

 concentric circles. In a section there are at least two such systems, the muscles 

 bang divided by a lateral partition into dorsal and ventral halves. There are 

 also smaller groups of muscles related to fins, gill arches, jaws, etc., but of much 

 smaller size, derivatives from the larger mass. Reference has already been 

 made (|) 1 1 1) to the modification of muscles into electric organs. 



The brain shows the low position of the class in the slight development of 

 the cerebrum. This is especially true of the teleosts (fig. 548), in which, in 

 place of a cortex, there is only a thin epithelial layer (Pall), what was formerly 



