June 13, 1889] 



NATURE 



157 



the bottom in proximity to the anemones fixed on the 

 rocks, and running the risk- of being themselves captured 

 for food, find in the inshore waters in summer the larval 

 PeachicB in great numbers conveniently attached by 

 the mouth to the little Hydromedusa; {ThauTnantias henii- 

 sphcerica and T. melanops) which occur in swarms in mid- 

 water. Moreover, the somewhat larger young food-fishes 

 {2 to 3 inches) show the same liking for the Coelenterate 

 group, by browsing on the zoophytes {Obelia geniculatd) 

 which cover the stones and rocks with feathery tufts, yet the 

 zoophytes are not much the worse for this treatment, for 

 they by and by shoot afresh, and clothe the area once 

 more with dense forests. The rapidity with which such 

 zoophytes grow is remarkable, though we must remember 

 that in some cases the old stock naturally dies off after 

 having produced swarms of pelagic young. 



Under this rich food, the young fishes grow apace ; 

 head and eyes, mouth and accessory organs, body and 

 fins — all rapidly increase ; and the little fish, hatched in 

 the spring, say from March to May, is soon in what is 

 known as the post-larval stage — that is, has lost its yolk- 

 sac, has assumed a more or less uniform tint, and has gill- 

 fringes and teeth. It is about a quarter of an inch long, 

 and is both active and intelligent, the large head and 

 large eyes of the young food-fishes being at this stage 

 specially conspicuous, and in marked contrast with such 

 as Coitus. The marginal fin is quite continuous at a 

 quarter of an inch, and the lancet-like termination of the 

 caudal end of the body is noteworthy. 



About this time the ventral fins of the young fishes first 

 make their appearance, for hitherto they have managed to 

 do without them. Moreover, these fins in some, such as 

 the rockling and ling, undergo remarkable development, 

 forming in the latter (Fig. 7) a pair of great ventral wings, 

 conspicuously coloured yellow ; yet in the adult (a ground- 

 fish) they attain no greater dimensions than in the cod, 

 both having at a certain stage soft, free filaments or tacile 

 processes at the tip. The ventral fins in the post-larval rock- 

 ling (Fig. 12) are ccinally large, the distal half being black, 



Fig. 12.— Post-larval rockling (enlarged). 



SO that at first sight the little fish when captured seems to 

 possess a great ventral spine on each side. In the post- 

 larval gurnard again, the huge pectoral fins form a drapery 

 for the entire body when folded back, only the tip of the 

 tail extending beyond them (Fig. 1 3). They are indeed pro- 



Moreover, their fondness for a minute reddish Copepod 

 {Calanus finmarchicus), which occurs in myriads around 

 them, gives the region of the stomach a faint pinkish hue 

 from the translucency of the tissues. By and by, however, 

 pigment appears, foreshadowing in the cod those peculiar 

 squares which give the sides, at a somewhat later stage, 

 their tessellated or tartan-like aspect. Besides, they are 

 found nearer the bottom of the water, so that they can be 

 captured in a naturalist's trawl with a fine gauze bag at 

 the end. There is, therefore, a downv/ard tendency as 

 the little fishes get older and stronger, and thus in many 

 cases a parallelism exists between them and the minute 

 forms on which they prey, for the t.gg~, rise on deposition 

 toward the surface, where the helpless larvae (or newly 

 hatched young fishes) also often occur, and then they seek 

 the lower regions of the water as their size increases. 



There is much that is wonderful in such a life-history, 

 especially in the metamorphoses or changes of form 

 undergone by many of our best fishes, such as the flat 

 fishes (Pleuronectid^), which come out of the egg just like 

 a haddock or cod, with an eye on each side, yet in after 

 life have both eyes on the same side, as in Fig. 2. Nothing 

 like this occurs in any of the higher vertebrates. Gradually 

 during growth the body of the fish increases in depth (Fig. 

 14), the right or left eye passes over the ridge of the back 

 to the opposite side (Fig. 15), while the creature, hitherto 



Fig. 14. — Young " witch" {Pleuronectes cynoglossus) in the third stage 

 (enlarged). 



Fig. 13. — Post-larval gurnard (enlarged). 



portionally as large as in the southern flying gurnards, but 

 in these the fins reach full development only in adult life, 

 while in the young stages they are comparatively small — 

 exactly the reverse happening in the grey gurnard of our 

 seas. The presence of the broad arches of pigment on 

 the pectorals of several forms, such as the present species, 

 green cod, and armed bullhead, is also an interesting fea- 

 ture. We have not yet read the riddle of all these 

 changes, but in the ling the great ventral fins are probably 

 connected with its roaming or pelagic life, and this ex- 

 planation would also suit in the case of the rockling, both 

 in their mature state seeking their food on the ground. 



The little fishes at this stage are still more or less 

 translucent, except in the region of the eyes, which are 

 silvery, and on the parts where the pigment occurs. 



Fig. 15. — Young "witch" at a later stage, the left eye just appe.-iring on 

 the ridge of the head (enlarged). 



I pelagic, sinks deeper in the water and exhibits a tendency 

 to lie on the side from which the eye has passed, and 

 which gradually loses its dark pigment so as to become 

 white.^ It finally reaches the bottom, taking up its resid- 

 ence amongst the sand or sandy mud, and lying with the 

 two eyes and the coloured side up, the white underneath. 

 i The mode by which the eye travels round has been a 

 I fruitful source of discussion with scientific men, and 

 I amongst these the names of Steenstrup, Malm, Schiodte, 

 \ and Alex. Agassiz abroad, Wyville Thomson and especially 

 i Traquair in our own country, are well known. The fact 

 i is, two methods exist in Nature : in the one the eye travels 

 over the ridge of the head, as just described in the 

 flounder ; in the other it traverses the soft and yielding 

 tissues of the tiny fish, and so gains the other side. In 

 Plagusia, the species in which the latter remarkable 

 change occurs in the post-larval stage, the general tissues 

 are so transparent that the creature in a glass vessel can 

 only be noticed by the two apparently disembodied eyes, 

 or by the gleam of light caused by its movements ; and 

 before the change ensues in its eyes it can look obliquely 

 through its own body and see what passes on the other 

 side.'"* 



' The tardy disappearance of the pigment in some forms is interesting. 



* Alex. Agassiz, Proceed. Americ. Acad. Arts. andSci., vol. xiv. p. 8, 1878. 



