248 THE FOOD OF ANIMALS 



plants known as Diatoms, and diminutive animals belonging to 

 the group of Foraminifera, in which the body is usually invested 

 by a calcareous shell. Like Cuttle- Fishes and Snails the Tusk- 

 Shells are provided with a rasping organ, not capable, however, 

 of being protruded so as to act upon large food-bodies. It is 

 probably employed in breaking up the small organisms secured 

 by the capture-filaments. 



BIVALVE MOLLUSCS (LAMELLIBRANCHIA) 



The BIVALVE MOLLUSCS are all omnivorous, and their food 

 is obtained by the agency of ciliary currents. The Freshwater 

 Mussels (Anodon and Unio) already described in vol. i, p. 328 

 will serve as a first example. One of the most striking charac- 

 teristics is the great reduction of the head, which can hardly 

 be said to be present at all. The development of this region 

 is proportional to the intelligence exhibited, and it contains 

 the highest parts of the central nervous system, together with 

 the most important sense organs. Intelligence is closely related 

 to the necessity for finding and seizing food, and there is every 

 reason to believe that the bivalves are descended from forms 

 which engaged in an active search for food, and were possessed 

 of a fairly well developed head. Animals in which food is pro- 

 vided in a mechanical way, without the necessity for individual 

 effort, always tend to become more or less degenerate, and this 

 appears to have been the case with bivalves, in which the head 

 has become more and more reduced, mainly, it would seem, in 

 consequence of the ciliary mode of feeding. The Mussel's body 

 is invested in two shelly plates, situated right and left, and lined 

 by flaps of the body- wall (man tie- flaps), between which is a large 

 mantle-cavity into which the body hangs down. The lining of 

 this cavity is clothed with cilia, and there are two large gill-plates 

 on either side, also richly covered with cilia. The mouth is a 

 transverse slit situated at the front end of the body, and is 

 entirely unprovided with anything comparable to jaws. On 

 either side of it are two sensitive flaps (labial palps), between 

 which is a food-conducting groove. These palps are also covered 

 with cilia. Observation of a feeding mussel shows that it is 

 obliquely buried in the mud, with the hinder-end projecting and 

 the shell slightly open. Two apertures between the mantle-flaps 

 are visible, a larger one below, into which water-currents con- 



