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 life 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 like the lobster, to forms which can 
only walk like the crabs, these showing many very 
interesting adaptations to their relatively sedentary 
life. 
The majority of the annelid worms are littoral, 
relatively few being adapted to the pelagic life. 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 liberated, 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, 
