TEGUMENTS OF FISHES. 



547 



362 



Cycloid scale, xxn. 



due to the presence of small round scales, about ^th in. diameter, 

 with concentric and radiating lines : they are set deep in the 

 derm. In the Sand-eel (Ammodytes), the 

 scales are proportionally larger, and one 

 margin rises from the derm and pushes out- 

 ward the portion of epiderm covering it : the 

 dermal depression is limited to the opposite 

 margin, and is deeper than in the Eel. The 

 free part of the scale retains the reticular 

 structure ; in the imbedded part the areola? 

 are obliterated in the direction from the centre 

 to the circumference ; the radiating lines preserve their distance, 

 but, being united by cross fibres close set, the structure appears 

 to be laminated. The majority of flexible scales present the 

 same pattern of concentric and radiating lines: the concentric 

 lines are the finest, most numerous, and constant ; they repeat 

 the contour of the scale, and with most regularity at the anterior 

 imbedded and covered part, where growth chiefly takes place, 

 the stages of which are marked by these lines. The ' nucleus ' or 

 beginning of the scale is usually excentric, fig. 362, n. The ra- 

 diating lines, fig. 362 and 363, r, r, are larger and fewer : they 

 are most numerous in the Loach ( Cobitis), are sometimes confined 

 to the forepart of the scale, fig. 363, or may be absent (Salmo): 

 they are furrows. The parts of the scale-margin between the 

 ends of the radiating lines usually project in different degrees 

 from a slight convexity, as in figs. 362, 363, to the form of pro- 

 cesses. The latter are most common at the anterior implanted 

 border of the scale (Esox) : in many fishes the opposite or free 

 border has numerous tooth-like processes, and 

 similar parts may project from the adjacent 

 periphery of the scale in two or more rows. 

 Such scales with a comb-like free border, fi<r. 

 363, t, characterise the fishes thence called 

 ' ctenoid : ' where the free border of the scale 

 is rounded or simply undulated, fig. 362, it 

 characterises the * cycloid ' fishes of Agassiz. * 

 The seat of chief vascularity and greatest 

 activity and variety of developement is at the 

 periphery of the derm, between it and the 

 epiderm. Here are formed the scales, constituting an imbri- 

 cate covering of the body in most fishes ; but, in a few, contiguous 

 or scattered. According to their structure and shape, scales are 



363 



Ctenoid scale, xxrr. 



XXII. 



N N 2 



