ANIMAL LIFE IN THE SEA 167 



their sucker-bearing arms. The flat creeping surface 

 which forms the foot in most gastropods, such as peri- 

 winkles, whelks, and so forth, indicates the dependence 

 of these animals upon a substratum. The bivalves 

 often live buried in the mud, like Mya and Lutraria, 

 or in sand like cockles, or they are attached by a byssus 

 or tuft of silky threads like the common mussel, or they 

 bury themselves in the substance of the rocks like 

 Pholas and Saxicava. Many of these forms, however, 

 have pelagic larvae. 



While many of the Crustacea are pelagic, like the 

 copepods or water-fleas, and various forms of prawns, 

 others, like crabs, are adapted for hfe on the bottom. 

 Indeed, we can trace among the shore Crustacea the 

 gradual acquirement of the ground-haunting habit, 

 leading by various stages from a form with the power 

 of swift swimming Uke the lobster, to forms which can 

 only walk Uke the crabs, these showing many very 

 interesting adaptations to their relatively sedentary 

 Kfe. 



The majority of the annehd worms are littoral, 

 relatively few being adapted to the pelagic hfe. Curiously 

 enough, however, not only have most of them pelagic 

 larvae, but certain forms become temporarily pelagic at 

 the breeding-season. The most striking example of this 

 is the palalo worm of various islands in the Pacific. 

 This worm lives among the coral reefs, but at certain 

 seasons a portion of the body containing the sexual 

 elements is Hberated, and leads a brief free-living life 

 at the surface, before death occurs. The liberation 

 of these sexual portions takes place at night, simul- 

 taneously in thousands of worms, so that the whole 

 surface of the water becomes filled with the animals, 

 which off Fiji, the Friendly Islands, and elsewhere, 



