GKOWTH AND NESTS OF FISHES. 611 



compensate for the chances that may intervene to prevent the 

 contact of the milt. 



116. Growth and Nests of Fishes. When developement has 

 stamped the Fish with its specific characters, growth proceeds at 

 various rates and to different degrees, according to the species 

 viz., from the size of the Stickleback to that of the Shark 

 of thirty-five feet long (Selache maxima). Carp, Pike, and some 

 other Fishes, which may live in ponds or lakes under circum- 

 stances favourable for continuous observation of the same in- 

 dividual, show that growth is not definitely arrested as an adult 

 character ; few Fishes, perhaps, can be called ( full-grown ' in 

 the sense in which the term is applied to warm-blooded Verte- 

 brates : but, after attaining the average size characteristic of the 

 species, individuals under favourable circumstances continue to 

 increase, though very slowly, in size. Growth is accompanied in 

 many species by changes of colour, in some by a greater propor- 

 tional size of the head, or by elongation or curvature of the mandi- 

 ble, or by increased length of a rostral prolongation sword or 

 saw : other special weapons, as the dorsal spines of Cestracionts, 

 File-fish, and Dog-fish, and both dorsal and pectoral spines of Sheat- 

 fish, acquire length and hardness, or dentate borders, in the course 

 of growth. External sexual characters are assumed, as shown in 

 the form and structure of the ventral fins in some Osseous Fishes 1 , 

 in the growth of the f claspers ' of Plagiostomes, and of the mar- 

 supial folds or pouches of Lophobranchs. In the Dolphin ( Cory- 

 ph&na), the cranial crest and fore-part of the dorsal fin gain so 

 much proportional height that young individuals of even two 

 feet in length were referred by Cuvier to a distinct genus 

 (Lampugus)? 



In a few instances the changes accompanying growth amount to 

 a metamorphosis. The edentulous state of the young Lamprey, 

 and the semicircular form of the upper lip, are exchanged for the 

 suctorial multidentate mouth, shown in fio-. 277. The external 



o 



branchial apertures enlarge, and the furrow in which they at first 

 open disappears. The perfect form of the Lamprey is not attained 

 until the fourth year. During half or two thirds of that time, the 

 growing Petromyzon presents a form which passes as that of a 

 distinct genus of Cyclostomes (Ammoccetes), 3 The Leptocephali 

 are probably larva? of some larger known fish : they have never 

 been observed with roe or milt : the same may prove to be the 

 case with Branchiostoma. 



' In almost all the Teleostomes the body of the young is more 



1 cccxxxvin. 2 CLXXIV. ii. p. 405. 3 cccxxvn. p. 323. 



R R 2 



