and the Endoplirarjinal System o/" Astacus. 69 



diving movements make it probable that the prevailing pulls 

 of these muscle-bands are for the bending of the abdominal 

 region. But altliough the greater part of the apodeme thus 

 slopes backwards, there are indications of a slight anterior 

 pull in its diamond form. 



Turning now to Astacus, we find very pronounced apo- 

 demes dorsally to the limbs, and in the line between them. 

 These are clearly homologous with the apodemes of Ajius^ 

 and originate as part of the segmental constriction between 

 the limbs. How they came to be drawn in we have learnt 

 from Ajms ; they were originally the points of attachment of 

 the segmental constrictions to the ventral longitudinal muscle- 

 bands inherited from the original Chatopod Annelid, and 

 retained in Ajnis. This origin is no longer apparent in 

 Astacus. Internally these endopleurites give off two branches, 

 one running posteriorly to the endosternite belonging to the 

 next posterior constriction, and one running in towards its 

 own endosternite. The former alone of these is, as above 

 described, developed in Apus, and has been handed on to 

 Astacus ] but whence came the anterior branch? 



A trace of this anterior branch is, as we have seen, present 

 in ApiiSf and might be easily developed if it were to be sub- 

 mitted to any strong anterior pulls of the longitudinal muscle- 

 bands. This is clearly what has happened in Astacus^ and 

 we have abundant evidence of such persistent anterior pulls 

 in the compression of the anterior ventral region. The effort 

 to bring forward the four anterior pairs of legs as maxilli- 

 pedes and forceps must have meant, for many generations, a 

 strong contraction of the ventral longitudinal muscles con- 

 necting the somites to which these limbs belong. To this, 

 then, 1 attribute the development of the anterior branch of 

 the endopleurite. 



The parallel between Apus and Astacus is, however, by no 

 means complete. The most anterior of the folds forming the 

 endophragmal system of Astacus cannot be called either an 

 endopleurite or an endosternite, because it is one deep con- 

 tinuous furrow only interrupted by the sternal canal. It 

 occurs between the 2nd maxilla3 posteriorly and the para- 

 gnatha and mandibles anteriorly. So capacious is it that 

 the 1st maxillcC may be said almost to spring from the bottom 

 of it. It is, in fact, nothing more than the sinking in of the 

 region of insertion of these limbs. 



Apus again supplies us with a complete explanation of this 

 phenomenon. The KSections 1-7 show us a very deep 

 fold behind the underlip, which runs in above the insertion 

 of the 1st maxilla. It is so pronounced that it forms one of 



