70 Mr. H. M. Berncavd oti the Apodemes o/Apus 



the principal supports of the " sternal plate " *. It runs in 

 further tlian any of the other lateral apodemes of Apus. I 

 consider the fold of some importance, and due not so much to 

 the bending round of the five annelidan segments to form the 

 crustacean head f, as to the forcing back of the underlip in 

 order to bring the mandibles in front of it. The counterpart 

 of this fold may be seen in a small apodeme in front of the 

 mandible, due to the forcing forward of the latter in front 

 of the underlip (fig. 2 a). 



This very pronounced fold beliind the underlip in Apus 

 very nearly coalesces with another less pronounced apodeme 

 between the 1st and 2nd maxiKffi. The fold between these 

 limbs is very deep, as is also that between the 1st trunk-limb 

 and the 2nd maxilla (Section 1). I am inclined to attribute 

 these to the bending round of the segments to form the head. 

 It is clear, therefore, that we have in Apus^ just behind the 

 nnderlip around the insertion of the 1st maxilla, an area of 

 subsidence, which, if it sank, would infallibly draw this limb 

 down with it, so that it would then spring from the base of 

 the fold. 



In Astacus this subsidence has actually taken place, and 

 the cause of it is not far to seek. The bringing forward of 

 the anterior trunk-limbs as maxillipedes necessarily com- 

 pressed the region of the body between these limbs and the 

 mouth, with the natural result that any tendency of the skin 

 to form folds in this region would at once be taken advantage 

 of, and the fold would become deeper. That this is the true 

 account of the origin of this fold in Astacus can still be made 

 out from an examination of its structure. It shows its com- 

 position out of two apodemes, the anterior of which, as in 

 AjniSj is much the more pronounced, bending forward at its 

 proximal end into a strong horn-like prolongation, which is 

 clearly the homologue of the stout apodeme mai'ked h in the 

 sections. If this is the case, then the sinewy tissue joining 

 the folds of the two sides across the middle line and thus 

 bridging over the sternal canal is the rudiment of a sternal 

 plate like that of Apus and Limulus. 



Following on this first fold of the endophragmal system of 

 Astacus is a somewhat complicated arrangement of folds and 

 ridges. The chief complication seems to be due to the fact that 

 the endopleurite between the first and second trunk-legs (or 



* This is commonly called the endosternite, but, having already 

 (following Huxley) used that term for the ventral apodemes, t here use 

 an alternative term for the sinewy mass to which the ventral longitudinal 

 bands are attached anteriorly. 



t Vide, ' The Apodidse,' pp. 10 et seq. 



