1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1137 
eggs and later in the period only 2 to 3. The oviducts are very 
conspicuous, and present dark tubes containing unripe eggs. The 
abdomen of the females is strongly curved and often, owing to 
pressure of the egg-sack, bent upwards at a right angle with the 
cephalosome. Yery often I have found females with remnants 
of an earlier egg-sack attached to the abdomen, at the same time 
having oviducts filled with unripe eggs. By this we learn that 
a female may produce more than one brood. From 30th September 
to 16th November the species gradually disappears. On 15th 
October the oviducts contain no more unripe eggs, and females 
with egg-sacks are rare. The va-sa efferentia of the male have 
no spermatophores ; a few males were found on 16th November but 
no females with eggs. The last individuals are observed on 23rd 
January, when the species totally disappears, new individuals again 
appearing in spring 1 904. 
As far as I have been able to make out, D. minutus has only 
one kind of egg, and I have never seen any egg beyond the 
gastrula stage. With regard to the colour, we find two kinds of eggs, 
one being red and uncleft, the other gray and cleft to the gastrula 
stage. First I supposed the shell of the former, which is of a 
yellow colour, to be a little thicker than the hyaline shell of the 
gray eggs, but later I found intermediate stages between these. 
Furthermore, as I have never found nauplii in the time from 14th 
July 1903 to 16th April 1904, I suppose that all of the eggs are 
resting eggs. 
Ekman (1904, p. 103) arrives at the same result with regard to 
the eggs of D. laciniatus , denticornis , and laliceps. 
Owing to the quality of the material it is impossible to give 
a correct conspectus of the life cycle of D. minutus ; in this 
respect it is most unfortunate that the collections have not been 
carried on after 30th June 1903, when the nauplii had begun to 
appear. 
I feel inclined to believe that the species has only one generation, 
which is hatched in April-May and dies out in January. This 
generation is derived from resting eggs which have hibernated in 
deep water either on the bottom or suspended in the water. 
My interpretation of the life cycle may perhaps be correct, inas- 
much as Marsh, in 1897 as well as in 1903 (p. 22), after a most 
PROC. ROY. SOC. EDIN. — YOL. XXY. 72 
