and the Endophragmal System o/Astacus. 71 



niaxillipedes) lias been drawn forward dorsally to, and a little 

 in front of, that between the 1st niaxillipede and the 2nd 

 maxilla. We can only ex))lain this by su})posing that the 

 niusele-pull on the first endoplcurite was very small, while it 

 was very strong on the second and following endopleurites. 

 This explanation is borne out by the fact that^ in Astacus, no 

 anterior branch of the first endoplcurite is developed. This 

 is easy to understand if we refer to Section 5. In this we 

 see the change that has to take place to give us the trans- 

 position of the 1st and 2nd endopleurites in the endophragmal 

 system of Astacus. In order to bring up the trunk-limbs to 

 act as maxillipedes, the muscular contraction must act on the 

 apodemes behind them, not in front of them. Hence we 

 find that, while the endoplcurite marked d has nearly re- 

 tained its original position, the endopleurites j^,r7, /<, &c. have 

 been drawn into positions y, g^, h', &c. indicated on the 

 Section (5) by dotted lines. The endosternites have been 

 also affected, those corresponding to d and f having been 

 drawn very close together. 



In addition to the longitudinal compression repeatedly 

 referred to above, the anterior part of the thorax of Astacus 

 has been subjected to considerable lateral compression, also 

 due to the transformation of trunk-legs into mouth-parts. 

 This has naturally forced the transverse segmental constric- 

 tions into folds, which are still visible in the buttress-like 

 backward prolongations of the endosternites, and perhaps 

 also in the sternal canal. This lateral compression of the 

 thorax in Astacus, forming a keel along the sternum, very 

 marked in the lobster, is in interesting contrast to what has 

 taken place in Branchipus, where the sternum is bent in 

 exactly the opposite manner, i, e. upwards, leading to a sepa- 

 ration of the two longitudinal halves of the ventral cord. 



Tenij)ting as it is, it is hardly necessary to show how the 

 ])Osterior ])ortions of the endophragmal system of Astacus 

 may be accounted for. They afford abundant evidence of 

 the strong muscular contractions which originally compressed 

 the thoracic somites. 



1 have thus, 1 think, made it very clear that Apus, with 

 its powerful ventral longitudinal muscle-bands and its flexible 

 skin, supplies us with a complete explanation of the endo- 

 phragmal system of Astacus, in which the ventral muscle- 

 bands, except those specialized for moving the tail, have 

 almost entirely disappeared, and in which the deep folds of 

 the skin which the now vanished ventral muscles once called 

 into existence have become permanent calcified ridges, 

 fastened together by sinewy connective tissue. 



