54 CLASS PISCES. 



along the middle dorsal line of the trunk, and continued round 

 the tail on to the ventral surface as far as the anus. It may 

 persist in this form, but as a rule it becomes broken up into a 

 variable number of dorsal fins, a caudal fin which consists of a 

 dorsal and ventral part, and an anal fin between the ventro- 

 caudal and the anus (Fig. 32). The unpaired fins are almost 

 always supported by the so-called fin-rays or dermotrichia. 

 These are horny fibres of the dermis {Elasmohranchii), or bony 

 rods {Teleostei, Ganoids, Dipnoi) which may be segmented, and 

 more or less soft and flexible (Malacopterygians) or stout and un- 

 segmented(Acanthopterygians). These dermotrichia are absent 

 only in Marsipohranchii. They are composed of two closely 

 approximated halves, and are carried except in the case of the 

 ventro-caudal fin, by the somactids or radialia. These are carti- 

 laginous or bony rods, placed as a rule in the basal part of the 

 fin-fold, and between the muscles of the back. They do not 

 necessarily correspond in number with the vertebrae. They 

 are usually segmented into two or three pieces, to the distal of 

 which the dermotrichia are attached. The basal piece is some- 

 times called the axonost ; in the Teleostei it is known as the 

 interspinous bone, because it occurs between the spines (neural 

 or haemal) of the vertebrae. The second piece is sometimes 

 known as the haseost. In a few fishes (e.g. Dipnoi), the 

 somactids articulate with the spines of the vertebrae. 



The strong spine-like anterior fin-ray often found in Teleosteans and 

 bony Ganoids is formed of bone. In Elasmohranchii, the strong spines 

 which are sometimes found in connection with the fins are tooth-like in 

 structure. 



The dermotrichia are of three kinds. * 



1. In Elasmohranchii and Holocephali they are unjointed, occasionally 

 branched, fibrous rays of a horny consistency and without osseous tissue : 

 these are called ceratotrichia. Similar dermotrichia are found in the 

 larval fins and at the edges of the adult fins of Teleosteans and Ganoids : 

 in this case they are called actinotrichia. They are more numerous than 

 the somactids. 



2. In adult Teleosteans and Ganoids the fins have jointed, branched, 

 bony dermotrichia developed between the actinotrichia and the skin. 

 They are supposed to be modified scales, which they sometimes resemble, 

 and are called hpidotrichia. They correspond in number with the 

 somactids except in the cartilaginous Ganoids, in which they are more 

 numerous. 



3. In the Dipnoi the dermotrichia have been called kamptotrichia 



* Goodrich, " Dermal fin-rays of Fishes," Q. J. M. S., 1904. 



