The sense-organs and perception of fishes. 249 



fish perceives. For if wire rabbit-netting or straight wires are 

 gently lowered while the fish is at the other pai't of his circle, he does 

 not avoid them on returning the first time. After colliding with 

 such an object, though very gently, the fish seems to lose its balance, 

 and does not swim upright for some seconds after, lying over gene- 

 rally towards the left side. If the wires are left in place the fish 

 does not again run into them, but swims in a reduced circle. The 

 sensibility of these fishes to movements in the water must be ex- 

 ceedingly delicate ; for if a straight wire is put in the path of a blind 

 fish it will be avoided if the finger only is kept on the top of the 

 wire, though it does not avoid it if the wire is standing by itself. 

 The same is true of pollack which had become blind from disease 

 (apparently of the nature of cataract, to which fishes in captivity are 

 very liable). It has been held by some observers that the sense- 

 organs of the lateral line are of importance to the equilibration of 

 the animal. Apart from the difficulties presented by the structure 

 of these organs, which closely resemble taste-buds, it must be held 

 that the case of the flat-fishes is practically conclusive against this 

 view. For in the flat-fishes not only do the lateral lines retain the 

 same position relative to the symmetry of the animal that they occupy 

 in other fishes, but in those fishes (e. g. the dab, &c.) in which the 

 course is peculiarly curved, this curve occurs equally on the upper 

 and under surfaces. But as the flat-fishes swim in a plane at right 

 angles to that of ordinar}'- fishes the two lateral lines come to lie in 

 the same vertical planes, and can therefore hardly be supposed to 

 assist in equilibration. The fact that a fish in which the lateral 

 nerve has been severed is unable to swim uprightly scarcely bears 

 on this question, for almost any severe injury upsets the equilibrium 

 of a fish. 



Shoaling. — It was suggested to me by Professor Lankester that in- 

 quiry should be made as to the manner in which fishes keep together 

 in shoals, and especially whether they follow each other by sight or 

 otherwise. The only shoaling fish which was living in quantity in 

 the tanks at Plymouth is the grey mullet. By day the whole shoal 

 of about fifty little ones stays together more or less. Sometimes it 

 divides into two or three shoals, but they run closely together if 

 alarmed.* At night they lie on the surface of the water, and seem 

 not to swim about as a body, nor are their heads all pointing one 

 way as they generally are by day. The shoal seems at no time to 

 have any leader, but will sometimes follow the front fish until one 



* This instinct of packing together when afraid seems to be general among fishes which 

 move in shoals. Mr. Dunn tells me that the proverbial phrase, " as close as hakes in a 

 hoop," is derived from the fact that such fishes huddle together when surrounded by a net. 

 Shoals of pilchard, herring, «fec., also pack together when attacked by sea-birds. 



