188 SUB-CLASS (AND ORDER) TELEOSTEI. 



there is no trace of eyes, optic nerve or olfactory nerve, and in 

 which the supposed luminous organs have the form of two 

 broad laminae on the upper surface of the head, and in other 

 deep sea forms in which the eyes are imperfect (e.g. the 

 Pediculati) they can only be of use as lures. 



In the endoskeleton the primitive cartilage is largely replaced 

 by bone, but some cartilage, varying in amount in the different 

 forms, may persist. 



The vertebral column is usually completely ossified and consists 

 of amphicoelous vertebrae (Fig. 34). The vertebrae are con- 

 nected by articulating processes placed on the neural arches. 

 In the trunk the centra carry transverse processes, which are 

 directed outwards, and ribs which are articulated to the centra 

 or to the base of the transverse processes (Fig. 34). In the tail the 

 centra carry complete haemal arches, which enclose a canal 

 containing the caudal artery and vein and are prolonged like 

 the neural arches into a median spine. In some forms a pair of 

 small bony rods the inter-muscular bones, are attached to the 

 centra near the neural arches. 



The vertebrae are arcicentrous, the notochordal sheath remaining 

 thin, but the skeletogenous tissue develops very little cartilage being 

 rapidly replaced by membrane bone in the centra as well as in the arches. 



In most Teleosteans the end of the vertebral column is bent 

 dorsalwards, and is unsegmented, though the notpchordal 

 sheath is ossified to form a bony urostyle. The haemal arches 

 of this part of the vertebral column persist in a modified form 

 as the hypural bones, which carry the dermotrichia of the ven- 

 tral part of the caudal fin (p. 55). In such cases the tail 

 though symmetrical externally, is internally asymmetrical, 

 and is said to be homocercal (see pp. 55, 56). In a few forms 

 (Gadidae, etc.), the end of the vertebral column is not bent 

 dorsalwards, and the tail-fin is symmetrical internally as well as 

 externally (diphycercal). In these fishes the dermotrichia of 

 the ventral part of the caudal fin are carried by interspinous 

 bones, and it seems highly probable that the true tail-fin has 

 atrophied completely, as it has in some Heteromi, Syngnathidae, 

 etc., in which the tail tapers to a point and is without any trace 

 of a caudal fin, and has been secondarily replaced by a backward 

 extension of the dorsal and anal fins (p. 55). Such tails are 

 therefore secondarily diphycercal. 



