1867.] Nerve Structure and Force. 159 



body, and a very large mantle, which so envelopes the body that 

 there is a spacious cavity in which both oral and anal orifices are 

 entirely concealed. Most are supplied with a calcareous covering 

 termed the shell. The nervous system consists of a number of 

 scattered ganglia, united by filaments with the oesophageal ganglia; 

 there is a distinct circulatory apparatus ; air-cells or branchiae for 

 respiration, and in many instances organs of vision. The mode of 

 life of these animals, and the offices which they perform in nature, 

 may be studied by the habits of the Solen ; and the nature of the 

 gasteropoda by the structure of the Paludina, or the Buccinum. 



The oblong form of the bivalve razor-shell is well known ; also 

 the structure of its foot, which enables it, by change of form into a 

 hook or a form of globular anchor, to sink or to rise in the soft 

 sand. The Solen is essentially a sand-borer, penetrating to a depth 

 of a foot and a half to two feet, rising for its food, and burrowing 

 out of danger, and mostly maintaining its vertical position until 

 death overtakes it, when its calcareous covering remains. The light 

 sandy colour of the shell resembles the soil in which it lives ; its 

 cutting shape best enables it to divide the soft sand ; its scattered 

 ganglia diffuse sensation over the several parts of an animal re- 

 quiring no active influence of centralized nerve force, but rather a 

 lower power generally diffused over its irregular shape. 



The scattered ganglia of the nervous system of the invertebrata, 

 and the absence of a well-defined brain, constitute an arrangement 

 by which the organic functions of the body may be best sustained, 

 without the exposure of the animal to the perceptions of pain. 

 The movements excited in its search after food must, in all pro- 

 bability, be reflex, and due in great part to the physical action of 

 the rays of light ; or currents caused by ciliary movements, bringing 

 objects within its reach, which, when touched, are seized, swallowed, 

 and digested. The act of deglutition brings the food within the 

 influence of the secretion of a liver; but no regular periods of 

 hunger prevail, and the animal can live long without taking food 

 at all. 



But ample provision is made for increase and procreation, and in 

 many instances by more than one process in the same individual. 

 The external senses consist chiefly of sight and touch, but the power 

 is very feebly developed, so long as animals are fixed to rocks and 

 other objects ; on the other hand, in the invertebrata of flight, such 

 as Inseda, the eyes are large and compound. Thus it is arranged 

 that the rate of increase in regard to these animals is not under 

 the disturbing influence of accident or passion ; that the supply is 

 constant and equal to the requirements of the earth, while the 

 important changes which are effected by their death in countless 

 myriads, either as trainers of rocks, or as a supply of food to larger 

 and more vigorous creatures, takes place without pain or suffering, 



