X . EXTERNAL CHARACTERS. 



fish (plate xlv) the anterior dorsal fin of the fry becomes converted into 

 free spines in the adult. The black pomfret, Stromateus niger, when young 

 has ventral fins present, causing it to have been placed in a distinct genus, 

 Apolectus stromateus. "While in the paired fins Ave find in the pelagic 

 tunny (plate xxxvi) that the pectorals become elongated with age, and 

 Gill observes that in the apodal or forms without ventral fins the body is 

 eel-like or elongated. I have remarked that a variety of form exists in our 

 ten-spined stickleback (vol. i, page 245, plate lxviii, fig. 4), in which these 

 latter fins are entirely wanting. 



Another mode in which fins change with age is owing to the spines, as of 

 the dorsal fin, not augmenting in length so rapidly as the soft rays, conse- 

 quently they may be comparatively shorter in the adult than in the young. 

 Even the soft rays in mature fish are commonly less in their proportionate 

 height to the entire length of the specimen than they are in the immature. 

 The same thing occurs in respect to anal spines, the second being sometimes 

 the longest in the immature, but becoming -shorter than the third in the 

 mature, which appears most frequently to take place when the second spine 

 is the strongest, augmenting in thickness while the third increases in length. 

 Occasionally there is an excess of a spine and a deficiency of a ray in the 

 dorsal fin, the anterior one_ of the latter having assumed a spinous character, 

 a not infrequent occurrence among the Sparidee : or several articulated rays 

 may be similarly changed. In some forms 'the number of rays, as in 

 the Cottidas, would seem occasionally to decrease as the species is found 

 further south. Spiny-rayed fish, preying upon their weaker neighbours, 

 appear to be more numerous in seas than in fresh waters, while in our inland 

 waters members of the carp and salmon families usurp their place, these 

 hitter being provided with articulated fin rays. 



Fin rays when broken may again unite, if lost they may likewise be 

 occasionally reproduced, but often in an incomplete manner. Injuries to 

 the caudal portion of the body sometimes cause remarkable changes in the 

 form of the fin, thus in a sole (vol. ii, p. 40) will be found the description of a 

 specimen in which the caudal fin in being reproduced has become continuous 

 above and below with the vertical fins. In an elongated Coilia of the 

 Indian seas I have several . times seen that a forked caudal fin has replaced 

 the last fourth or fifth of the caudal portion of the body, which probably 

 had been lost by accident. 



Not only do the component portions of a fin become greatly altered, but 

 the fin itself may be very dissimilar to what we find in a typical perch or 

 carp. Thus the dorsal fin is entirely absent in the electric eel, Gymnotu*, 

 of South America : it may be a long single fin, the front portion be spiny, 

 as in the sea-perch (plate v), or with only soft rays, as in the sole (plate cvi) ; 

 or the fin may be in two portions, the first being composed of spines, and 



