DISTEIBUTION IN DEPTH. 151 



The distribution of the MoUusca in Depth has been investigated 

 by MM. Audouin and Milne-Edwards, M. Sars, and Professor 

 E. Forbes. By these observers the sea-bed is divided into four 

 principal regions : — 



1. The Littoral zone, or tract between tide marks. 



2. The Laminarian zone, from low water to 15 fathoms. 



3. The Coralline zone, from 15 to 50 fathoms. 



4. The deep-sea coral zone, 50 to 100 fathoms or more. 



1 . I'he Littoral zone depends for its depth on the rise and fall 

 of the tide, and for its extent on the form of the shore. The 

 shells of this zone are more limited in their range than those 

 which are protected from the vicissitudes of climate by living 

 at some depth in the sea.* In Europe the characteristic genera 

 of rocky shores are Littorina, Patella, and Purpura; of sandy 

 beaches, Cardium, Tellina, Solen ; gravelly shores, Mytilus ; 

 and on muddy shores, Lutraria and Pullastra. On rocky coasts 

 are also found many species of Haliotis, Siphonaria, Fissurella, 

 and Trochus ; they occur at various levels, some only at the 

 high-water line, others in a middle zone, or at the verge of 

 low-water. Cyprcea and Co7ius shelter under coral-blocks, and 

 Gerithium, Terehra, Natica, and Pyramidella bury in sand at low 

 water, but may be found by tracing the marks of their long 

 burrows. (Macgillivray.) 



2. Laminarian zone. — In this region, when rocky, the tangle 

 [Laminarici) and other sea-weeds form miniature forests, the 

 resort of the vegetable feeding moUusks — Lacuna, Bissoa, Nacella, 

 Trochus, Aplysia, and various Nudihranchiata. On soft sea-beds 

 bivalves abound and form the prey of Buccinum,, Nassa., and Natica. 

 Erom low-water to the depth of one or two fathoms on muddy 

 and sandy shores, there are often great meadows of grass- wrack 

 [Zostera) which afford shelter to numerous shell-fish, and are 

 the haunt of the cuttle-fish and calamary. In tropical seas, the 

 reef-building corals often take the place of sea-weeds, and 

 extend their operations to a depth of about 25 fathoms. They 

 cover the bottom with living verdure, on which many of the 

 carnivorous mollusks feed, while some, like Ovt^Zwrnand Purpura, 

 browse on the flexible Gorgonice. To this zone belong the 

 oyster-banks of our seas, and the pearl-fisheries of the south ; 

 it is richer than any other in animal life, and affords the most 

 highly coloured shells. 



Some of the littoral shells, like Purpura lapillus and Littorina rudis, have no 

 free-swimming larval condition, but comn>'"»'"e life as crawlers, with a well'developed 

 shell. Their habits are sluggish, and tneir diffusion by ordinary means must be 

 exceedingly slow. 



