68 Mr. H. M. Bernard on the Ajyodemes o/Apus 



hoping that some one would attempt a detailed deduction of 

 Astacus from the Apodid^. As a contribution to such an 

 attempt, I propose here to show how Aims supplies us witli a 

 full explanation of the endophragmal system of Astacus. 



Apodemes, or inward foldings of the skin, are very plenti- 

 ul in A2JUS, and their origin is in all cases clear. 



We liave, first of all, the segmental constrictions, which 

 are naturally obliterated in the stretched regions of the 

 body, but very marked in longitudinally com])ressed regions. 

 For our present purpose we confine our attention to the con- 

 strictions on the ventral surface anteriorly. In some speci- 

 mens, according to the state of contraction at death, these are 

 very deep and throw the ventral cord into a series of waves 

 (c/ PI V. fig. 7). 



In Astacus the moving forward of the anterior trunk-limbs 

 to function as mouth-parts or maxillipedes, and the conse- 

 quent longitudinal compression of this region of the body, 

 necessarily caused these constrictions to form high fixed 

 barriers across the inner ventral surface. Over these barriers 

 the ventral cord had at first either to stretch, or to form a 

 series of arches. Each barrier has, however, in time been 

 cut down in the middle line, so that the cord has come to 

 lie in a groove along the ventral surface, known as the sternal 

 canal. The subsequent arching over of the canal by sinewy 

 matter is a secondary arrangement which also receives its 

 explanation in Apus, as we shall presently see. 



So far, this explanation of the origin of the endosternites 

 (as the two halves of these constrictions are called) is simple 

 enough. The origin of the endopleurites (or endotergites as 

 they are sometimes called) is not quite so evident. 



Taking first the endopleurites between the trunk-limbs, we 

 find that in Apus, in the anterior part of the body, where the 

 limbs are developed, the two ventral longitudinal muscle- 

 bands are attached to the segmental folds between the limbs. 

 These points of attachment naturally tend to be drawn in- 

 wards by the action of these muscle-bands. Further, as the 

 pulls of the muscles are in the longitudinal direction, the 

 folds naturally acquire the diamond-shape shown in the 

 tangential sections (figs. 5, 6). In these apodemes the dorso- 

 ventral diagonal is the natural direction of the segmental 

 constriction ; the longitudinal diagonal is due to the pulls 

 of the muscle-bands. By comparing the sections we find that 

 each of these apodemes in Aj^jus is pulled backwards as far as 

 the segmental constriction posterior to that to which it really 

 belongs. This is easy to understand ; the length of the 

 posterior region in Ajms and the use made of it for sudden 



