in the Fish Gallery of the Indian Museum. 19 



often interlock with one another ; (4) Placoid scales, 

 which are either bony granules or tooth-like bony- 

 plates with a covering of enamel : placoid scales are, 

 in fact, often not different, either in form or composi- 

 tion, from the teeth of the fishes that possess them. 



In most fishes (see the labelled Sea-perch in Case 8) 

 a row of scales along either side of the body is speci- 

 ally modified to form what is known as the lateral line. 

 This is a subcutaneous groove that usually extends from 

 the back of the head to the tail, and the scales that 

 cover it are channeled and perforated and are specially 

 provided with nerve-endings, so that the lateral line is 

 an organ of sense. 



A series of specimens of the various sorts of scales is 

 to be found in Case 7. 



The scales of fishes are usually imbedded in distinct pockets of the skin, 

 and are protected (as also is the skin of those fishes that have no scales) by 

 a copious layer of slime, or mucus, secreted by small cutaneous glands. 



The integument of Fishes is almost always coloured. The colours fade 

 both in dried and spirit specimens, but in this Gallery an attempt has been 

 made, as far as possible, to represent the natural colours by painting. 



The tints are commonly dull with a metallic sheen, but many marine 

 species are most gorgeously coloured. 



Fishes that live on sandy and shingly bottoms are often speclded and 

 marbled in various shades of yellow and brown on the upper surface (or, in 

 the case of Flat fishes, on the upper side) ; while those that haunt reefs 

 encrusted with a variety of sea-weeds and zoophytes are often most wonder- 

 fully mottled, or else have the skin decorated with frond-like tags and tassels 

 and fringes. By thus harmonizing with the colour and form of surrounding 

 objects they escape the notice of their enemies and avoid scaring their prey. 



Moreover, many fishes can, within certain limits, temporarily change colour 

 to suit changes in their surroundings : this is brought about by varying the 

 amount of contraction and expansion of the special cells — known as chro- 

 matophors, or colour-carriers — in which the colouring-matter is contained. 

 For instance a fish may, as in the case of several species of Platycephalic 

 be dependent for its colouration on two pigments — yellow and dark 

 brown, which in ordinary circumstances are arranged in bands across the 

 body. If, however, the fish be resting on a dark surface, the yellow cells 



