18 SALMONID^ OF BRITAIN. 



The skiu, excluding that of the head, is scaled, the scales being cycloid and 

 partially imbedded at their bases, the posterior or external portions of each over- 

 lapping its neighbour like tiles, and consequently they are termed imbricate. They 

 have their free edges directed backwards, thus preventing their being any 

 impediment in the water when swimming. Scales increase in size in proportion 

 with the growth of the body of the fish : consequently the adult has the same 

 number as the young, and also they are imbricated to much the same extent. The 

 number of rows existing along the lateral line is subject to considerable variation, 

 while those in a lino passing from the hind edge of the adipose dorsal fin 

 downwards and forwards to the latei-al-line afford one of the most constant 

 characters by which these fishes may be discriminated, there being from 10 

 to 11 in S. salar and its varieties, and 13 to 15 in the various forms of British 

 trout. 



Although fishes do not periodically shed or change their scales as birds do their 

 feathers, still such as have been lost by injury can be renewed. Special organs of 

 touch are developed in fishes in several different manners. Even papillae, as round 

 the mouths or on the lips, of some forms are highly sensitive, and as has been 

 shown by Leydig and others they are abundantly supplied with nerves. 



The lateral-line consists of a series of tiibes along either side of the body, and 

 is often known as the muciferous system ; but although from it mucous is 

 excreted, it is essentially an organ of sense, and as such is variously modified, 

 while the inside of the canal is lined with epithelial cells, and here nerves ramify, 

 and terminate in an expansion. 



Intimately related to the tegumentary system and the composition of scales is 

 the subject of colour. Among the finny tribes the display of colour may be of the 

 most varied description, some tints being permanent, many transient, and others 

 again of periodical occurrence. Certain of these coloui's are due to the influence of 

 light* while they may vary in the same fresh- water species owing to the character of 

 the water they inhabit ; for should the latter be opaque and muddy, fish as a rule 

 are darker than when obtained from localities where it is clear, those in running 

 streams are generally lighter and brighter than when from stagnant pools, or 

 such as are from shallow pieces of water than from those that are deep, while if 

 captured in dark caverns they are frequently destitute of both coloiir and vision. 



Age and season likewise exercise an influence in this respect, as do also 

 the state of health and temporary local emotions. In the very young but few 

 markings or colours are present, but these rapidly develop themselves by the 

 time, or even before, the first breeding season has been reached, when the brilliancy 

 of the individual has often attained to its maximum. This nuptial adornment is 

 generally acquired a short time prior to this period, subsequent to w^hich it usually 

 disappears. Some see in the markings on these creatures grounds for assuming 

 the probability of the descent of many forms from some common ancestral 

 progenitor. Thus, among the young of the members of the Salmonida', dark 

 bands or bars down the sides are almost universal, being evidently hereditary 

 throughout the family. f These bars, as well as the black and red spots, are almost 

 lost in anadromous forms when they are in a condition to migrate into salt water, 

 for then they assume their silvery smolt stage. In large rivers it is rare for bands 

 to be retained in adult trout, while most of the black and red ocellated spots as a 

 rule remain, although instances have been recorded in which they have entirely 



* Pouchet and others have pointed out that the changeable tegumentary colours of fish 

 depend more especially upon two conditions. Firstly, we have iridescence effected by an interference 

 with the rays of light, owing to the presence in the scales of thin plates or ridges, and in these 

 forms the tints change with great rapidity in accordance with the angle at which they are viewed : 

 such lamellar coloviring is common among insects, Crustacea, and some fishes — it is beautifully 

 seen in the Dolphin, Corijphwna, and the scale of a common herring furnishes a good example. 

 Secondly, a distinct anatomical element, as chromatophores or colour-sacs, which are often highly 

 coloured, may be present and capable of changing their form under special influences, which are 

 apparently directly connected with impressions of colour received by the eye and brought about 

 by the reflex action of the nervous system. 



_ t This same banding of the young is seen in some species of the horse-mackerel or Varanx, 

 flying-fish or Exocatiis, the gar-fish or Belone, &c., in which they are usually a sign of the 

 iinmaturity of the individual. 



