VERTEBRATA : FISHES. 483 



The order Teleostei comprises almost all the common fishes \ 

 and it will be unnecessary to dilate upon their structure, as 

 they were taken as the types of the class in giving a general 

 description of the Fishes. It may be as well, however, to recapit- 

 ulate very briefly some of the leading characters of the order. 



I. The skeleton, instead of remaining throughout life more 

 or less completely cartilaginous, is now always more or less 

 thoroughly ossified. The notochord is not persistent, and the 

 vertebral column, though sometimes cartilaginous, consists of 

 a number of vertebrae. The bodies of the vertebrae are what is 

 called " amphiccelous " that is to say, they are concave at 

 both ends. It follows from this, that between each pair of 

 vertebrae there is formed a doubly-conical cavity, and this is 

 filled with the cartilaginous or semi-gelatinous remains of the 

 notochord. By this means an extraordinary amount of flexi- 

 bility is given to the entire vertebral column. In no fish except 

 the Bony Pike (which belongs to the order Ganoidei} is the 

 ossification of the vertebral centra carried further than this. 

 The skull is of an ^extremely complicated nature, being com- 

 posed of a number of distinct cranial bones ; and a mandible 

 or lower jaw is invariably present. 



II. The anterior and posterior pairs of limbs are usually, but 

 not always, present, and when developed they are always in 

 the form of fins. The fins may be supported by " spinous " or 

 "soft" rays, of which the former are simple undivided spines 

 of bone, whilst the latter are divided transversely into a num- 

 ber of short transverse pieces, and also are broken up into 

 a number of longitudinal rays proceeding from a common 

 root. (The fishes with soft rays in their paired fins are 

 termed " Malacopterygii " those with spinous rays, " Acan- 

 thopterygii") 



III. Besides the paired fins, representing the limbs, there is 

 a variable number of unpaired or azygous integumentary ex- 

 pansions, which are known as the " median fins." When fully 

 developed (fig. 261), they consist of one, two, or three fins on 

 the back the " dorsal " fins ; one or two on the ventral surface 

 the "anal" fins; and one clothing the posterior extremity 

 of the body the " caudal " fin. The caudal fin (fig. 253, A) is 

 set vertically, and not horizontally, as in the Whales and 

 Dolphins ; and in all the bony fishes its form is " homocercal " 

 that is, it consists of two equal lobes, and the vertebral 

 column is not prolonged into the superior lobe.* In all the 



* Though to all appearance symmetrical, the tail of the bony fishes is 

 in reality generally unsymmetrical. The appearance of symmetry is due 

 to the bony spinal column terminating in the centre of a wedge-shaped 



