218 Natural History of Molluscous Animals: — 



Art. III. An Introduction to the Natural History of Molluscous 

 Animals. In a Series of Letters. By G. J. 



Letter 13. On their Food and Digestive Organs. 



In reference to the present subject, I shall divide mollus- 

 cous animals into three classes : — 1st, those which take their 

 food in a liquid form, or suspended in water ; 2dly, those which 

 are more properly carnivorous; and, 3dly, those which feed 

 on vegetable matters. 



To the first class all the MoUusca tunicata belong, and the 

 tenants of the bivalved shells. There is no one of either of 

 these extensive tribes which is furnished with any organ 

 adapted to the capture or arrestment of prey, or with jaws or 

 teeth to tear and masticate it ; and, as the greater number are 

 immovably fixed to one spot for life, or are only capable of 

 such motions as raise or depress them in their furrows, they 

 are necessarily content to await what moist nutriment is brought 

 within reach of their lips by the waves and currents of the 

 circumfluent waters. The Mollusca tunicata have the power 

 of enlarging the capacity of their large branchial sac ; and it 

 is probable that, during this action, a portion of water rushes 

 in, with all its contained animalcules, which serve for the food 

 of the individual. I have found, in the sac of some of the 

 compound and smallest species .(^Icy^neae), myriads of very 

 minute corpuscles, which I believed to be entomostracous 

 insects ; and Savigny, who has frequently made the same 

 observation, has found, in the sac of species of the same 

 tribe, crustaceous insects of a higher order and greatly larger 

 dimensions. The latter, however, as Cuvier thinks, may have 

 entered against the will and to the prejudice of the molluscum ; 

 for he has observed the delicate texture of the viscera torn 

 and ruptured by such rude ingesta. {Mem., xx. p. 14.) 



Of the Mollusca tunicata there are two families : one, Al- 

 cyoneae, or the social, in which numerous individuals, generally 

 of very small size, are united together, and, as it were, im- 

 mersed in a common somewhat gelatinous mass ; and another, 

 Ascidia?, or the solitary, in which every individual is single 

 and separate, and of much greater magnitude [see figs. 24. and 

 25. p. 129, 130. of the preceding Number]. In both of these 

 families, the mouth is a circular aperture, raised a little above 

 the surface of the common integument or sac, and is capable 

 of being shut or opened, more or less widely, at the pleasure 

 of the animal. The rim of it is sometimes plain, and some- 

 times cut into four, six, or eight equal segments ; and within 

 the orifice there is, in very many of them, a fringe formed of 

 one or two rows of delicate cilia, which I have observed, in 

 the Ascldia rustica, to be in constant and quick vibration 



