BRANCHIFEROUS MOLLUSCA. 285 



young, clustered in hollows of rocks that were many feet 

 ahove the highest tides. Still, their respiratory organs are, 

 as they ever have 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 ala- 

 crity ; and why their eyes have not risen to a higher eleva- 

 tion than the base of the tentacula, that they might scan 

 the landscape and avoid its perils. The habits of the Chi- 

 tonidae are similar to those of the Littorinae. " These 

 animals," says the Rev. Mr. Guilding, " frequent the rocks 

 and stones of the sea-coast, and are distributed nearly over 

 the whole globe. Many of the species are constantly under 

 water, while others ascend above low or even high- water- 

 mark, spending the day exposed to the hottest sun, or select- 

 ing a resting-place which is only occasionally moistened by 

 the rude and restless surf. In Chitonellus and Cryptocon- 

 chus there are certain minute organs on the zone, which 

 bear a strong resemblance to the spiracula of the annulose 

 animals. From their habit of quitting the watery element, 

 like many of the Turbinidae, I once supposed that the 

 organs for the aeration of the circulating fluid might be of 

 a compound nature (pulmono-branchiati). It is, however, 

 far more probable (as in the case of some crustaceous genera 

 which I am now investigating) that this process is capable 

 of a diurnal or a temporary interruption, or that the bran- 

 chias, so long as they are kept moist, and shielded from 

 atmospheric influence, may perform their functions, though 

 much more slowly." 



It is probably this capableness of protecting the branchiae 

 from the drying influence of the air that enables even such 

 mollusks as habitually live submerged to survive their re- 

 moval from the sea for a very considerable time, so that they 

 may be thus carried to the greatest distances without injury, 

 while any attempt to carry them, or to preserve them, in 

 sea-water would have failed, without a care to renew the 

 water that could seldom be bestowed. The death in experi- 

 ments of this kind does not ensue from the abstraction of 

 oxygen from the water, but from its corruption ; and hence 

 you will find that, if not mechanically prevented, the animal 

 will creep from the poisoned bowl to expose itself to a 

 slower death in an element which it cannot breathe. But 

 what you would little anticipate to hear, fresh water is to 

 these pelagic molluscans even a deadlier poison, and quicker 



