232 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



is falling, but the proportion will diminish far less rapidly 

 than in the spring and summer, for the incubation period 

 during the month of minimum sea-temperature is at least 

 four times greater than during the month of maximum sea- 

 temperature. If we could allow for this variability the curve 

 of spawning of fig. 2 would fall more rapidly than it appears 

 to do, that is to say, the spawning period would not be so 

 prolonged as the figure indicates. 



With the exception of Ehrenbaum's well-known work 

 no good investigation of the life-history of the North European 

 shrimp has been made, so that we do not know how long the 

 incubation period is, nor how it varies with the temperature. 

 Further, the stages of development are not so well-known 

 that we can say exactly at what time, prior to the time of 

 observation, an egg was spawned. By counting only new-laid 

 ova we could, of course, make an estimate of the variations 

 in the spawning time, and it was with this object that observa- 

 tions of the stages of embryonic development were made. 

 But it will be necessary to carry on these observations, first 

 of all, in an aquarium. 



According to Ehrenbaum there are two spawning periods 

 in the year with respect to the shrimps in the Heligoland 

 Bight, the first from the middle of April to the beginning of 

 June, and the second during October and November. It is 

 not impossible that there may also be two chief spawning 

 periods in Liverpool Bay, but the observations made so far 

 do not appear to show that this is the case. 



