438 TEXT-BOOK OF ZOOLOGY 



motion, it cannot, like a snail, for instance, go to seek for itself. Both 

 these requirements, therefore — viz., water rich in oxygen and its necessary 

 food — are carried to it by the current. 



To the blood which streams through the leaf-like gills, this current 

 gives up its oxygen in exchange for carbonic acid gas, whilst the minute 

 food particles which it contains (microscopic plants and animals, decaying 

 animal and vegetable substance accumulated in and floating on the top 

 of the mud) are conveyed to the so-called labial tentacles — two small, 

 leaf -like processes on each side, right and left, of the mouth. These 

 processes also are covered with cilia, which, by their vibrating move- 

 ments, conduct the food particles to the mouth, which lies somewhat 

 concealed in front of the foot. 



The water, after being thus deprived of its useful constituents, is 

 next expelled, together with excreted matters, through the upper smaller 

 opening at the posterior end of the animal, which is accordingly called 

 the efferent aperture, in distinction from the other opening, which 

 is described as the afferent aperture. Both openings are gutter-like 

 grooves of the mantle edges, which by apposition form small tubes. 

 If the edges of the mantle be touched ever so lightly, they at once 

 contract, showing that they are extremely sensitive. The tentacles at 

 the afferent aperture also are highly sensitive to tactile impressions. 

 (Why this portion of the body especially ?) 



(c) Organs absent in the Fresh-mater Mussel. — As it lives exclusively 

 upon microscopic particles, the fresh-water mussel lacks all those organs 

 which are concerned in the division of the food — e.g., jaws and radula— 

 such as we find in gastropods. Moreover, as the animal does not go in 

 search of its food, it is devoid of feelers, eyes, and likewise of a head 

 carrying these organs. (Some lamellibranchs, however, which lead a 

 more active life, are provided with a fairly large number of eyes 

 developed on the projecting edge of the mantle.) 



(d) Why the Fresh-water Mussel is restricted to an Aquatic Existence. — 

 The atmosphere does not contain floating food particles in sufficient 

 quantity to provide nourishment for an animal. The number of micro- 

 scopic animals living in the air is inconsiderable, whilst microscopic 

 plants are entirely absent, and such decaying animal and vegetable 

 substances as are whirled about by air-currents soon fall to the ground 

 again. Nor would it be possible for a sessile animal to produce in a rare 

 fluid like the air an eddying current for the conveyance of food materials. 

 On the other hand, these conditions are exactly reversed in water, 

 which is so much denser than air, and so rich in suspended food 

 materials. Hence fixed, or, at any rate, to a large extent, sessile animals 

 like the fresh-water mussel can only exist in water, and accordingly all 



