72 ESSENTIALS OF ZOOLOGY 



the bivalve Mollusca are remarkably consistent. The mouth 

 is anterior, but the food is introduced into the body by an 

 opening at the posterior end. This opening may be removed 

 still further away by the inhalent opening being carried out 

 on a siphon, e.g. Mya, Lutraria. In some cases, as Scrobi- 

 cularia and Teredo, the inhalent tube and the exhalent tube 

 are independent and are capable of active movements, and may 

 be retracted and protracted. The food, which is nearly always 

 suspended material, is carried by ciliary action with a large 

 volume of water into the mantle or pallial cavity. The water 

 is filtered by the gills, and the food matter, entrapped in mucus, 

 is carried by ciliary action to the region of the labial palps, 

 and so to the mouth. The gills therefore are not merely 

 branchial in function, and it has been suggested that their 

 blood may take up dissolved food as well as oxygen from the 

 water. It is obvious that the success of the bivalve depends 

 upon the supply of food in its neighbourhood, for the mussel 

 has only a limited range, and besides it is necessarily 

 gregarious. 



The adductor muscles close the shell by active contraction 

 of the fibres, and in the case of bivalves exposed to the air 

 the shells have to be kept tightly closed for a long time, and 

 in that of oysters and mussels packed for market for many 

 weeks. It would obviously be a severe physiological effort to 

 maintain the contraction by a constant succession of stimuli, 

 and it has been found that the contracted fibres are locked 

 automatically in the contracted state, and can only be released 

 when a stimulation reaches the locking fibres. 



Bivalves lead as a rule a sedentary life, and are so protected 

 by the valves as to be only subconsciously interested in their 

 surroundings. Some, as Pecten, are active, and in associa- 

 tion with the activity develop additional sense organs. They 

 are subject during the course of their years of life to a 

 summer increase in growth, in physiological activity and re- 

 production, and to a state of relative rest in the lower tempera- 

 ture conditions of winter, and such alternations are recorded 

 in the growth of the shell. 



In the young state Anodonta and its allies are liable to 

 considerable spread when parasitic on fish. In North America, 



