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LETTER XVII. 



ON THEIR FOOD AND DIGESTIVE ORGANS. 



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

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

 their food in a liquid form, or suspended in water ; 2ndly, 

 those which are more properly carnivorous ; and, 3rdly, 

 those which feed on vegetable matters. 



To the first class all the Mollusca 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 num- 

 ber 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 w^aters. The 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 

 and microscopic vegetables, which serve for the food of the 

 individual. Sir J. G. Dalyell says that the food of the 

 Ascidiffi " seems to consist of what may be eliminated from 

 muddy solutions. Quantities of mud suspended in water 

 are evidently absorbed and long retained ; they visibly fill 

 the intestinal cavities of those species whose transparence 

 exposes the interior. If the clearest sea-water be rendered 

 turbid, it is speedily purified by the secerning operation of 

 internal organs, serving to select the nutrition, while the 

 residue is rejected, to be discharged in rolls or cylinders." * 

 What they select from this mud are the infusorial vegetables 

 with which it is loaded. I have found in the stomach of the 

 Ascidiae, as also in the sac of some of the compound and 

 smallest species (Alcyoneae) myriads of very minute cor- 

 puscules, which the microscope showed to be Diatomacese 

 of various genera.f Lowig and Kolliker found that the 



* Riirc and Remark. Anini. Scot. ii. 140. 



t Dr. Dickie has given the following list of Diatomacese which he found 

 in the stomachs of different Ascidise taken near Aberdeen. 



