LIFE HISTORIES OF CERTAIN FRESH-WATER FISHES. 487 
the heart, and the blood can be seen in them flowing down- 
wards and forwards. No doubt they assist in the absorption 
of yolk as in the case of the Salmon alevin. 
In the batch of larvie which I kept under observation the 
yolk was not completely absorbed until after about three 
weeks, by which time the rays of the dorsal and anal fins were 
just becoming visible, and the rays of the caudal fin were 
developing beneath the end of the notochord which had not 
yet commenced to bend upwards. The little fish had gradually 
become thickly covered with dark chromatophores, but were 
still fairly transparent. Between 4 and 5 weeks after hatching 
the fish was easily recognisable as a Stickleback, although 
development was not complete. The caudal fin was in the 
midst of the heterocercal stage, the rays of the dorsal and 
anal fins were well developed and the spines on the back were 
visible as little protuberances in front of the dorsal fin. The 
larval fin had to a great extent disappeared. About a fort- 
night later most of the fish had taken on the adult appearance 
and metamorphosis was complete. 
These observations were made during the course of other 
work, so that it was not possible to devote much time to 
them. No efforts were made to keep the various species 
alive in the aquaria beyond supplying them liberally with 
cladocera as food. The Pike, Roach and Stickleback survived 
without any difficulty, and although my observations as a rule 
were not repeated, I reared the larvae of these three species 
on several occasions. The Perch never survived more than 
about 14 days; the complete absorption of the yolk sac being 
always the finale of its life-history in the aquaria. 
I only hatched the eggs of the Bream once, as only once 
did I obtain a ripe male and female at the same time. I should 
have expected the Bream to survive as readily as the Roach 
under similar conditions, but possibly my specimens were 
handicapped in some respect. 
The Sticklebacks and Roach lived together on peaceable 
terms although the former always seemed better fed than 
the latter. The Pike after their period of rest attacked and 
as a rule swallowed any other larva placed in their aquarium. 
