io THE COMMERCIAL SEA FISHES OF 



passes somewhat obliquely between it and the' upper layer 

 or epidermis. The basal portion of the scale consists of 

 mostly somewhat gristly laminae, while calcification is 

 developed from the upper layer of the dermis ; conse- 

 quently both the upper and lower layers of the skin assist 

 in the formation of scales. We find that scales may take 

 on many characters, as denticles in the sharks, osseous 

 plates in sturgeons ; while in bony fishes they may either 

 degenerate or give rise to structures which are very dis- 

 similar to typical scales, as seen in the plates and spines of 

 the plectognaths and pipe-fishes or Lophobranchii. 



Scales may either overlap, when they are said to be 

 imbricate, or they may be at a greater or less distance apart. 

 The most common forms of scales have received distinctive 

 names ; those which are more or less flexible and have 

 a spinate outer margin, as seen in the perch, are termed 

 ctenoid, or comb-like ; those in which the edges are smooth, 

 are known as cycloid, or circle-edged, as found in carps. 

 The second division of scales are as hard as teeth, due to 

 the predominance of bone earth, and consist of the ganoid, 

 which somewhat resemble the eve-tiles of a house, and the 

 placoid, or such as are provided with a spine. Although it 

 is frequently highly desirable to ascertain whether a fish 

 has cycloid or ctenoid scales, still there are many instances 

 in which they do not appear to be correlated to any impor- 

 tant functions. Thus among the gobies some have ctenoid, 

 others cycloid scales ; or both forms may exist on the same 

 fish, or even scales may be entirely absent, and all these 

 variations occur within the limits of a single genus. 



External colours in fish are often due to the influence of 

 light, while the tints even in the same species are found to 

 vary considerably in different localities, or when under 

 peculiar forms of diet. In some the colours may be due to 



