270 THE BIOLOGY OF THE SEA-SHORE 



The number of eggs laid at one time is not large and they 

 are carried about by the female in the brood pouch. Accord- 

 ing to Hewitt (1907), the ova of Ligia oceanica are large, oval 

 in shape, and contain a large amount of yolk. 



Growth of Crustacea. — Orton {op. cit.) has made 

 use of an ingenious method for determining the growth- 

 rate of some Crustacean forms. A wire basket of small 

 mesh is lowered in the sea and is entered by such young 

 animals as are small enough to pass through the narrow mesh. 

 Some of these remain and, feeding on the natural growth 

 which accumulates on the basket, increase in size and so are 

 trapped. From an experiment of this kind Orton concludes, 

 for instance, that the prawn {Paleemon serratus) becomes 

 mature and carries eggs within a year, and that the common 

 shore crab, Carcinus maenas, also attains maturity within a 

 similar period. 



The fact that many Crustacea are in process of transition 

 from an aquatic to a terrestrial life lends particular interest 

 to their reproductive habits. These show an interesting 

 analogy with the habits of Amphibia, inasmuch as while 

 the adult animal may be essentially terrestrial a return 

 must be made to the original habitat for breeding purposes. 



Crabs of the genus Geocarcimis , which live in sheltered 

 situations several miles from the sea, are said to make their 

 way in spring in immense numbers to the sea-shore where 

 breeding and spawning take place (Smith and Welldon, 1909). 

 A return migration occurs when this is completed. The 

 young pass through the normal larval stages in the sea and 

 then migrate inland. A similar return to the sea for breeding 

 purposes is also necessary in the case of the robber crab 

 {Birgtis latro). 



Other genera whose members are in process of gaining 

 a place on the land are Ocypoda and Gelasimus. 



Molluscs. — The sexes may be either separate as in most 

 Pelecypods, Mytilus, Cardium, etc., and nearly all Proso- 

 branchiata, e.g. Patella, Acmcea, Fissurella, Haliotis, TrochuSy 

 Neritina, Natica, Littorina, Rissoa, Hydrohia, Nassa, etc., 

 or united as in Opisthobranchiata and Pulmonata, e.g. Bulla, 



