STRUCTURE OF FISHES. 9 



flattened fibres running at right angles to each other; the outer layer, or epidermis, is full of large 

 star-shaped pigment cells, but devoid of scales. The Eel has a soft and thick epidermis. Below the 

 pigment layer are the narrow oblong scales, which are formed of a finely reticulate cartilage. In the 

 genus of Blennies named Zoarces there are circular depressions over the skin, due to minute round 

 scales embedded in the derrnis. Most osseous fishes possess flexible scales, marked with either con- 

 centric or radiating lines, or both combined. In these scales there is usually a nucleus, which may 

 be irregular. The radiating lines diverge from the circumference of this nucleus. These lines are 

 very numerous in the scales of the Loach (Colitis). The parts of the margin of the scale between 

 the radiating lines usually project in little convexities, and when the irregularities are limited to 

 one end of the scale that end of the scale is usually implanted in the skin. In many fishes the free 

 end of the scale is bordered with tooth-like processes. The surface of the body in most fishes is 

 lubricated with a thin layer of mucus, but in the Eel and Tench the mucous layer is thick. This fluid 

 is secreted by a canal which extends along the body, and has many ramifications among the bones of 

 the head, where it exudes through pores upon the cranium, face, jaws, pre-operculum, and through 

 tubes which perforate the scales along what is called the lateral line, usually distinguished on the 

 sides of the fish by a lighter or darker colour. Rymer Jones 

 remarks that after a fish has been dried in a napkin it soon becomes 

 covered again with mucus, which issues from the pores. In the Tunny 

 (T/ii/nnus thynnus), there may be seen beneath the skin, running the 

 entire length of the lateral line, a glandular organ, from which the little 

 tubes are given off to the lateral line. The mucous system, however, is 

 best developed in the Rays. In the genus Acanthoclinus there are several 

 lateral cainals which give off short tubes, which tunnel a way through 

 the scales as they pass onward. Sir Rich. Owen remarks that the 

 silvery and golden lustre of fishes is mostly on the surface of the scales. 



SCALES OF FISH. 



The silvery pigment known in commerce under the name of argentine 



. , ,. . A, Cycloid; B, Ctenoid; c, Ganoid; 



consists or very minute crystals of various earthy substances scraped . riacoid. 



from the scales, which often also occur upon some of the internal 



organs. The blue, red, green, and other bright colours of fishes are usually due to coloured oils, 

 which occupy cavities in the skin, and are capable of changing their position, so as to alter the 

 colour of the fish under the influence of excitement, or in harmony with the colour of the sea- 

 bed upon which the fish is living. Many fishes change colour after death. 



The scales of fishes consist of two layers. The lower layer resembles the fibro-cartilage 

 of the human body, while the upper layer contains cartilage cells similar to those which are seen in 

 the bluish cartilage covering a joint. The parts of a scale are defined in relation to the nucleus or 

 focus from which growth originates. The longitudinal lines which run out from this nucleus some- 

 times form furrows and sometimes perfectly closed tubes. The broad plates which form the armour 

 of the Pipe-fish are penetrated by canals, which all converge from the margins towards the middle f 

 the scale. The concentric lines of scales are found to originate in the development of new cells?, which 

 become filled with horny matter, and ultimately arrange themselves in concentric lines. Scales show, 

 when examined with the microscope, corpuscles, which are similar to those seen in bone. The 

 fibrous layer of the scale may easily be found by scraping off the external cellular lines and corpuscles, 

 when the fibres of the lower layer will be seen to cross each other at various angles. The growth of 

 the spines upon scales appears to be similar to the growth of teeth, for each spine is contained in a 

 distinct capsule or envelope. When the capsule is opened the spine can be easily removed from 

 it, but as the germ develops it acquires roots, and comes to consist of several layers. Professor 

 Agassiz, impressed with the differences of form in the scales, at one time believed that fishes might 

 be classified by means of them, and he proposed to divide the scales into four types : those which were 

 bony, and formed of a thick osseous layer, covered with hard transparent enamel, as in the 'genera 

 Lepidosteus and Polypterus, were termed Ganoids ; those dermal spines or tubercles seen in the 

 Thornback and many other cartilaginous fishes, which have a spine arising from a more or less 

 circular bony base, were called Placoid scales ; the scales which have the free margin more or less 

 comb-shaped were termed Ctenoid ; and those marked with a concentric structure were named Cycloid, 

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