.PHYSIOLOGY OF THE INVERTEBRATA. 397 



portion of the body is reduced almost to nothing, and con- 

 stitutes then a movable apron placed on the lower surface 

 of the body between' the limbs. 



In Astacus the ambulatory limbs are composed of seven 

 separate joints : the basal joint being the coxopodite which 

 is followed in succession by the following joints : — ^the basi- 

 podite, ischiopodite, meropodite, carpopodite, propodite, and 

 dactylopodite. 



The Mollusca. 



The movements in these animals are, as a rule, executed 

 by means of a muscular organ, termed the foot, which varies 

 greatly in its form, in accordance with the habits of the 

 animal. The foot consists of a mass of muscular fibres, run- 

 ning in various directions, by the contraction .of which its 

 movements are effected. In many of the Mollusca, the foot 

 forms a flat disc, which adheres to any substance to which it 

 may be applied, and thus, by the alternate contraction and 

 dilatation of its different parts, enables the animal to crawl 

 slowly along. In other forms, the foot is bent upon itself, so 

 that its sudden extension causes the animal to perform a con- 

 siderable leap {e.g., Gardium and Trigonia). This organ is 

 also the means by which the burrowing species bury them- 

 selves in the sand or mud ; and in those species which bore 

 in the solid rock, the foot is also called into requisition ; its 

 surface in these cases being covered with minute silicious 

 narticles, which assist greatly in the enlargement of its owner's 



my dwelling. 



. 3ut although most Mollusca possess a greater or less power 

 of locomotion, others are confined to a single spot, during all 

 but the earliest period of their existence, when they are free- 

 swimming organisms. In the non-locomotive Mollusca the 

 foot is either wholly undeveloped (e.g., Ostrea), or serves 

 merely to support a glandular organ, from which a chitinous 

 or shelly substance is secreted, which serves to attach the 



