1889.] on the Life-Mstonj of a Marine Food-fish. 397 



described iu the flounder ; in the other it traverses the soft and yielding 

 tissues of the young 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 jjasses on the other side.* 



Up to this stage in the life-history of both round and flat fishes it 

 will have been apparent that the etforts of man can have little efiect 

 on the vast multitudes of the eggs and minute fishes. His trawl 

 sweeps beneath them, or they are carried harmlessly through its meshes. 

 Not even in the case of a trawl blocked by a fish-basket and several 

 large skate are any likely to occur. No example indeed was pro- 

 cured in the trawling expeditions for the Commission under Lord 

 Dalhousie. The hooks of the liners are too large for the mouth at 

 this stage, and hence they escape capture. Their small size and 

 translucency also seem to afford j)rotection in the case of predatory fishes 

 of their own or other kinds, for they are rare, so far as present obser- 

 vation goes, in the stomach of any fish. Their great numbers are 

 doubtless kept in check by some means, and we know that even 

 jelly-fishes (e. g. PleurohracMse) are very fond of post-larval fishes. 

 It is only when they become somewhaf larger that they are preyed on 

 by their own and other species, and are swept up in thousands by the 

 destructive shrimp-nets on our sandy shores. 



While the little food-fishes are assuming the change of hue 

 indicated in the preceding pages, they in many cases seek the inshore 

 waters ; at least systematic use of the mid- water and other nets prove 

 that at certain seasons they are met with in large numbers at the 

 entrance to bays or off shore, and that a little later — in the case of the 

 cod from the first of June onwards — they are visible from the rocky 

 margins. The coloration in this species (cod) is now beautifully 

 tessellated (Fig. 6), and they swim in groups, often in company with 

 the young green cod, at the margin of the rocks at low water, and in 

 the little tidal bays connected with rock-pools. The latter are often 

 richly clothed with tangles, bladder-weed, red and green seaweeds, 

 and the green Ulva — amidst the mazes of which the young fishes find 

 both food and shelter, capturing the little crustaceans (Copepods, 

 Ostracods, and others) swimming there, and snatching the young 

 mussels and minute univalve mollusks from the blades of the sea- 

 weeds. To the zoologist few sights are more interesting than to 

 watch the little cod in these fairy lakes, as they swim in shoals 

 against the current, balancing themselves gracefully in the various 

 eddies by aid of their pectoral fins. In a mixed company, the young 

 cod are easily recognised by their coloration, and the reddish hue of 



* Alex. Agassiz, ' Proceed. Americ. Acad. Arts, and Sc, vol. xiv. p. 8, 1878. 



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