186 SUB-CLASS (AND ORDER) TELEOSTEI. 



The spines of the dorsal fin of Acanthopterygians can be raised or de- 

 pressed at will. In the depressed position the spines may cover one 

 another completely (homacanth), or they may be turned slightly to one 

 side or the other alternately (heteracanth). 



The skin usually contains pigment and may be very brightly 

 coloured. Many fishes possess the power of changing their 

 colour in a protective manner according to their surroundings, 

 and in almost all the dorsal surface is darker in colour than the 

 ventral. The body is usually covered with scales, overlapping 

 one another in such a way that the posterior part of the scale is 

 free and covers the anterior part of the next scale behind. The 

 scales are thin plates of bone imbedded in the dermis, and are 

 more frequently absent from the head and fins than elsewhere. 

 They are absent in most eels and in fishes with electric organs. 

 The epidermis is soft and contains many mucous and pigment 

 cells. 



The scales have a concentric striation and are of two kinds, cycloid 

 and ctenoid. In ctenoid scales the posterior free margin possesses denti- 

 culations which may extend on to the surface, whereas in cycloid scales 

 such denticulations are absent. In some cases the scales are enlarged 

 into great scutes (Siluridae, Lophobranchii, Plectognathi) and in Balis- 

 tidae they have the form of ossified papillae which project in a shagreen- 

 like manner. In Siluridae the scales may carry movably articulated 

 dermal teeth. 



The scales of most Teleostean fishes are thin calcified lamellae, without 

 bone corpuscles, lying in a dermal sac. They are surrounded by sclero- 

 blasts which, as in Lepidosteus, form them and add to them during growth. 

 The outer portion of the scales is structureless, the inner contains fibres. 

 In some forms, scleroblasts of the upper surface are included and become 

 bone corpuscles, so that the scales consist of an outer part showing ordin- 

 ary bone structure and an inner consisting of connective tissue impreg- 

 nated with calcareous matter by the scleroblasts. 



The spines which are present on the scales of some Teleostei, e.g. some 

 Acanthopterygians and Plectognaths, frequently contain a cavity which 

 suggests a pulp cavity. They may possibly be regarded as homologous 

 with the spines of Elasmobranchs, but without the enamel cap. The 

 spines of the hinder part of the ctenoid scales are due to the sculpture of 

 the surface and are not denticles. 



According to the recent and as yet (March 1904) unpublished researches 

 of Marett Tims, the scales of Gadus consist of a number of small plates of 

 structureless bone (vitrodentine) lying close together on a fibrous basis. 

 In the young state each plate carries a small spine which does not reach 

 the epidermis. 



The sense organs * (nerve eminences) of the lateral line are 



* F. Leydig, Integument u. Hautsinnesorgane der Knochenfische, ZooL 

 Jahrb. Anat., 8, 1894, p. 1-152. W. Collinge, Sensory Canal System of 

 Teleostei, Proc. ZooL Soc., 1895, pp. 274-299. R. McDonnell, Lateral 

 line in Fishes, Trans. P. Irish Acad., 129, 1862, pp. 161-187. 



