SPECIAL PROBLEMS OF SHORE LIFE 63 



when the animal withdraws into it. In some cases this 

 operculum is grooved on one side and serves as a brood 

 pouch in which the eggs undergo development. 



The capacity of shore animals for resisting desiccation 

 varies considerably, this capacity being in direct relationship 

 to the position occupied by the animal on the shore. 



The Littorinidae have progressed a considerable distance 

 on the road to a land life. Thus, L. neritoides lives in shaded 

 crevices just above high-tide mark. Its salt-water require- 

 ments are satisfied by dashings of spray and the moisture 

 consequent on its close proximity to the sea. Tropical 

 species of Littorina seem still more capable of dispensing 

 with moisture. Cooke (1895) has seen in various parts of 

 Jamaica, L. muricata living at the top of low cliffs among 

 grass and herbage, and at Panama he has taken three large 

 species of Littorina {L, varia, L. fasciata^ L. pulchra) on 

 trees at and above high-water mark. 



The following table of Colgan's (19 10) gives the maximum 

 period of survival of littoral mollusca on {a) exposure to dry 

 air, {h) immersion in fresh water, and {c) immersion in water 

 of treble the salinity of sea- water. 



Littorina neritoides 

 L. rudis 

 L. litter ea . . 

 Hydrobia ulvce 

 Purpura lapilius 

 Lasaa rubra (a bivalve) 

 Trochus umbilicatus 

 Littorina obtusata . . 



On exposure 

 to dry air. 



42 

 31 

 23 



9 



7 

 6 



days 



On immer- 

 sion in fresh 

 water. 



II days 



7 



7 



18 



4 

 o 



4 

 2 



On immersion 



in water of 

 treble salinity. 



6 days 



7 



6 



9i 



3 



o 



3 

 3 



As might be expected, capacity for resisting desiccation 

 is accompanied by greater tolerance of fresh water, but 

 Lascea rubra and Paludestrina stagnalis (Hydrobia idvce) 

 form exceptions which are not easy of explanation. 



In spite of the undoubted capacity of these littoral 

 molluscs for resisting drought and the importance of the 



