AND OTHER BRITISH PRAWNS. 119 



When the same pools were visited again in August all the 

 Eurytemora had disappeared, and the numbers of young Palce- 

 monetes were far less than of the larvae previously present. 



The marshes are occasionally flooded by exceptionally high 

 tides, which may sometimes occur in early summer. In such 

 cases the young must be washed out to sea and spread about 

 along the coast. If it were not for such floods, every pool would 

 become an isolated unit in which a struggle for existence of 

 extraordinary intensity would operate, and the circumstances 

 would be mo.st favourable for the establishment of local races. 

 How far the population of the various pools are in fact isolated 

 it is difficult to ascertain, but there are pools which are probably 

 not flooded by any but very exceptional tides such as occur at 

 intervals of several years. Probably, however, isolation in this 

 sense rarely, if ever, persists for very long, and the sea carries 

 the larvae up and down the coast, intermingling the various 

 populations. 



Breeding-period, &c. 



The breeding-period begins, in jSTorfolk, about the middle or 

 end of May and ceases about the end of July. Larvfe were first 

 met with this year near Yarmouth on June 16, all being in the 

 first stage and hatched since about four days, as most of them 

 moulted within the next two or three days. On June 17 zoseas, 

 nearly all in early stages, were found in great abundance at 

 Wells, so that about the middle of June may be regarded as the 

 main hatching-season. The records of the Maiine Laboratory at 

 Plymouth show breeding to begin there in April and to continue 

 till July. The eggs are cairied 5 or 6 weeks before hatching, if 

 I may judge from a single female kept in an aquarium, but pro- 

 bably both the rate of development of eggs and larvee would be 

 rather more rapid in natural conditions. After July I have not 

 found egg-bearing females, and larvse in early stages are not 

 met with. 



I have not found any direct evidence of females producing a 

 second brood, as is usually the case with L. squilla and L. longi- 

 rostris. Examination of large numbers of young taken in 

 September bears out the conclusion that second broods are the 

 exception. That second broods are, however, produced occasionally 

 there can be no doubt, for in some cases the young are found to 

 fall distinctly into two age-groups. For instance, in a pool at 

 Cley on September 6 it was possible to draw a clear distinction 

 between young of 17-22 mm. representing the first brood, and 

 others of 11 mm. or less, some of which were still in the second 

 post-larval stage and must have been hatched about the end 

 of July. 



Probably the production of a second brood depends a good deal 

 on local conditions, and is likely not to take place in such small 

 pools, which must be extremely overcrowded, whereas it may be 

 the rule in other and more favourable conditions. 



