CHAPTER V 

 SKIN, SCALES, AND SPINES 



Structure of skin. Dermal denticles of Selachians. Tail-spine of Sting Rays. 

 Saw-fishes. Scales of Bony Fishes: ganoid scales, cycloid and ctenoid 

 scales. Tubercles. Bony scutes. Armoured fishes. Bony plates, rings, 

 spines. Scales of Lung-fishes. Arrangement of scales, scale counts. 

 Axillary scales. Lateral line. Scale-reading and age-determination. 

 Replacement scales. 



The skin of a fish, like that of any other vertebrate, is composed 

 of two layers, a thin outer epidermis and an inner dermis. The 

 epidermis is made up of several layers of simple cells, of which 

 the outer are being constantly worn away by wear and tear, 

 and replaced by new ones budded off at its base. The dermis 

 has a more complicated structure, being made up of a thick 

 layer of what is known as connective tissue, with which are 

 mingled muscle fibres, clusters of fine blood-vessels, and nerves. 

 The inherent slipperiness of a fish's body is due to the presence 

 of a slimy mucous which is constantly being poured out in 

 large quantities by special glands situated in the epidermis, its 

 function being to minimise friction with the surrounding water 

 and to enable the fish to glide easily along. The slime excreted 

 varies greatly both in quantity, and probably also in com- 

 position, in different species. In some of the Lampreys the 

 glands are especially numerous, while a single Hag-fish (Myxine) 

 placed in a bucket of water will soon convert the fluid into a 

 thick mass of whitish jelly. 



In addition to the skin, there is generally an outer covering of 

 scales of one kind or another, generally spoken of as the exo- 

 skeleton, to distinguish it from the endoskeleton (skull, back- 

 bone, etc.). Where it is overlaid by scales the skin itself is 

 nearly always thin and delicate, but in those fishes without 

 scales, plates or spines it is strengthened in some way. Thus, 

 in the naked Cat-fishes {Siluroidea) it is thick and leathery, and 

 in the Sun-fish (Mola) the tough roughened skin is further 

 reinforced by an underlying layer of cartilaginous material two 

 or three inches in thickness. The curious Horse-fish (Agriopus) 

 of South Africa is unique in being able to cast off its skin 



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