216 



ICHTHYOLOGY. 



c. Dermal or Exo-sheleton. To this system the scales be- 

 long, each being secreted, 

 like a tooth, in a depres- 

 sion or pocket of the skin, 

 to which it adheres un- 

 der the cuticle. They are 

 very generally imbricated 

 with the posterior end ex- 

 truded and free, but the 

 skin usually extends over 

 the sm-face of the anterior 

 end to a greater or less 

 extent ; or the scale may 

 be entirely imbedded be- 

 neath the cuticle. Pro- Fig. 24. 



feSSOr Ao-assiz considered a. Cy.-lmrt s<-ale of Lampanycles (or M,,clo- 



o ptmm) rfspUii'lens, one ot the nocturnal 



the form of the scales to fcopdinm of tlio great group of Sabiio- 



bear so strong a relation to 



the rest of the ichthyic structure and the general economy 

 of a fish, that he founded upon it his primary divisions of the 

 class, of which he characterized four. The fish of his Cycloid 

 order have scales composed of concentric layers of horn 



scales (fig. 28); in others the teeth are more persistent, Introduc- 

 though the earliest, ^\hich are the nearest to the centre tion. 

 of the disk, are generally the smallest. There are some I^ermal 



Skeleton. 



Fig. 25. 

 .Scale of a Labrold. 



or bone, without spinous margins, and not covered by enamel. 

 He believed that a close relationship existed between the 

 absence of pimgent teeth on the scales, and the want of 

 spinous rays in tlie fins, and tliat 

 this order has a considerable corre- 

 spondence with the 3Ialacop/eri/f/ii ^/TxZj^ 

 of former ichthyologists. The 6V/- ? ^T v 



clo-Lahridw, however, and many 

 Gobiid(E, are exceptions to this 

 correspondence, and others exist. „. j^ 



Most commonly the Cycloid scales cycioia scale of 'a'adopsn mar- 

 axe imbricated ; occasionally they "'<"•""'■'■ "r Acantimpteryeian 



, I'll • 1 • with rndimentarv thoracic 



are placed side by side, in contact pectorals, it m'hahiis the 



b* „ ^ „ „i ■• • rivers of Sontliern Anstr.llia. 



ut not overlapping ; sometimes 



they are more remote ; and in Anguilla, the narrow, oblong 

 cycloid scales are in 

 groups lying at right 

 angles to each other, so as 

 to produce a kind of 

 lattice-work under the 

 cuticle. Some genera of 

 fishes with smooth scale- 

 less skins come into this 

 order. The Ctenoid 

 order of fish have horny 

 or unenamelled bony 

 scales, with spinovis teeth 

 on the posterior edges of 

 the layers like a comb 

 (figs. 27, 28, 29, and Fig. 27. 



^1 ^ Ac tli<3 cr>alf*c (rrnMr^^*^""'^ scale of Scatopfiauits mnllifasctatus, 

 d I j. AS tne scales grOVV „„„ „f j,,^ Squa,r,mu>ennis or (Jhatodonadd: 



their SpinuleS wear ofl", f'''"° ""O Australian seas. 



and in some species we observe no more than a single 

 series on the posterior edge of the last ftirmed layer of the 



Fig. 28. Fig. 29. 



Fig. 3H. Ctenoid scale of Gnlms ommatnriis. 

 ... 2il. Ctenoid scale of Psettus argeuitus, a Chajtodontoid. 



scaleless groups associated with Ctenoids. Scales of this 

 and the preceding order have most commonly grooves ra- 

 diating from the centre of the disk over that part which 

 remains in the cuticular pouch, and crenatures on the basal 



Fig. 30. Fig. 31. 



Scale from the head of Ctenoid scale of Lethrinus ct/no- 



Macronrits Austi-alis. citeilvx. one of the Norfolk Island 



Sparid<c. 



edge corresponding with tlie number of grooves. In both 

 orders, also, the most common circumscription of the scale 

 is circular or subcircular ; occasionally they are irregular or 

 oblique ; and the scales in one part of the fish difi'er from 



'.'-'i^ftf^ 



■ Fig. 32. 

 Scale from the lateral line of Macrourus Australis. 



those in another, in outline, and more frequently in size. 

 In some Labroids the largest scales are on the base of tlie 

 caudal fin, but in most of the osseous fishes the largest lie 

 towards tiie middle of the sides of the body. The Ctenoid 

 scales are mostly imbricated. 



