THE MOLLUSKS. 303 



lands, must have seen the common water-snails come floating down the current, lying on their 

 backs, their shells submerged, and the edges of their fleshy foot turned up on all sides so as 

 to convert that organ into a miniature flat boat. 



That the Moll asks, or, at all events, some of the species, possess the sense of hearing, is 

 tolerably evident from an examination of the structures. Near the nervous knobs, or ganglia, 

 as they are scientifically termed, of the head, are placed some little vesicles, each filled with a 

 transparent fluid, and containing a tiny knob, or spikelet, of chalky matter, very similar to 

 t the well-known ear-bones of fishes, and probably serving a similar purpose. These "bones" 

 appear to be perpetually in motion within their crystalline cell. 



The circulation of the Mollusks is tolerably defined, especially in the higher and best- 

 developed species, where the blood is urged on its course by a definite heart, and ramifies 

 through the body by means of well-developed vessels. In the lower forms, however, these 

 vessels can no longer be distinguished, and the blood circulates through a system of little 

 cavities distributed in the body. So completely is this the case, that many Mollusks can be 

 successfully injected, by introducing a line-nosed syringe at random into the body, and press- 

 ing the heated substance very gently into the system. 



The movements of the Mollusks are mostly performed by means of the mantle, and 

 through this structure the shell is secreted and molded into form. 



Of the secondary services rendered to man by the Mollusks we know but little, owing 

 to the localities in which the greater number of species live, their nocturnal or darkness-loving 

 habits, and their extreme dislike to intrusion. Several species, such as the pholas or burrow- 

 ing shell, and the teredo, so notorious as the ship-worm, are well known to be actively 

 injurious to man, by destroying the foundations on which his edifices are built, or the vessels 

 in which he trusts his life and property to the waves. Yet even these insidious enemies 

 may have their uses to man, and by destroying the wrecks on which many a noble vessel 

 might be driven and share the same fate, may be the salvation of costly property and invalu- 

 able lives. 



As to those which are known to be directly useful to mankind, it will be sufficient to 

 give a brief enumeration at present and to mention particulars when we come to the individual 

 species. 



Usefulness to man is, when reduced to its essence, the capability of affording him food, 

 and therefore the edible species must take first rank as regards use. The oyster is familiarly 

 known to rich and poor, the latter being often more practically cognizant of its value than the 

 former. It affords at once a refined luxury and a health-giving nutriment ; it can be eaten 

 uncooked, or opens a wide field for culinary art ; and it has the further advantage of being 

 very plentiful, very cheap, very accessible, and very easily preserved in a living state until 

 needed. The mussel is another largely consumed Mollusk, especially among the lower classes 

 who cannot afford to buy oysters ; and in some parts of the world is cultivated and bred in 

 millions, the ever-increasing numbers, together with the peculiar accommodation which they 

 require, threatening to obliterate many a natural harbor, and causing a well-grounded appre- 

 hension among ship-owners that their vessels may be deprived of their accustomed refuge by 

 means of this simple Mollusk. The scallop again, with its classically famous shell and coral- 

 red foot, is another of the edible species, as is the cockle, another well-known bivalve. Some 

 of the foreign bivalve Mollusks are considered as very great delicacies, among which the clam 

 takes a very high rank. There is also the huge giant clam, formerly rare, but now perfectly 

 familiar; which has to be cut away from the rock by hatchets, and whose contents are equiv- 

 alent to a large round of beef, very well flavored, but rather tough and stringy. 



The single-valved species furnish many edible examples, such as the whelk and the 

 periwinkle, so largely consumed by the poor, and even the cuttles are capable of affording 

 a tolerably good repast when properly dressed. As a general rule, however, the bivalves 

 are most esteemed, as they are not so fibrous in texture, and therefore not so tough as the 

 univalves. 



The shells of the Mollusks are also of much service to mankind. Putting aside the well- 

 known money cowry, perhaps the most infinitesimally divided currency in the world, many 



