LITTORINA. 355 



spots where they can be covered only at high water, 

 and I have seen myriads of them, when young, clus- 

 tered in hollows of rocks that were many feet above 

 the highest tides. Still, their respiratory organs are, 

 as they have ever been, branchial; nor does it seem 

 easy, on the Lamarckian hypothesis, to account for 

 their non-improvability ; why these shell-fish, so fond 

 of air, have not acquired, by their residence in it, the 

 lungs of the snail, and betaken themselves to the land ; 

 why their shells have not become lighter to enable them 

 to move with more alacrity ; and why their eyes have 

 not risen to a higher elevation than the base of the ten- 

 tacula, that they might scan the landscape and avoid its 

 perils/' The gill-plume is composed of from 45 to 60 

 strands or pectinations, which are very long, slender, 

 and close-set. Adanson appears to have considered the 

 Littorina hermaphrodite ; but, on his return home from 

 Senegal, he was undeceived in this respect by the great 

 botanist, Jussieu, who showed him that the sexes were 

 certainly distinct in the common European periwinkle. 

 Most of the species are oviparous, and deposit their 

 spawn on seaweeds, rocks, or stones; the eggs are 

 enveloped in a glairy mass, which is just firm enough 

 to retain its shape in the water, and adheres to the 

 nidus with considerable tenacity. Each egg has its 

 own globule of jelly, and is contained within an ex- 

 tremely thin and transparent membrane, so as to be 

 separated from the rest. They are hatched after a short 

 exposure to the water, air, and sun, and soon exhibit 

 the shells completely formed and occupied by the 

 ciliated fry. Some species are ovoviviparous or vivipa- 

 rous, and develope their spawn in the branchial cavity. 

 We find, therefore, in this genus, examples of both kinds 

 of propagation. The same fact has been observed with 



