326 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY 



complexity in the endophragmal system of the last five 

 thoracic somites. The endosternite and the endopleurite 

 of the same arthrophragm instead of being in the same 

 plane, as in the Macrura, may be situated at a consider- 

 able angle to each other, so that. it is difficult to believe 

 that they belong to the same segment. The fifth endos- 

 ternite is almost vertical, hut the succeeding endosternites 

 incline more and more forward until the last arthro- 

 phragm is practically horizontal. 



The endopleurites of each arthrophragm become fused 

 with the anterior face of the following arthrophragm, and 

 thus we have each somite divided into four chambers. 

 There is an outer chamber at each side lying between two 

 consecutive endopleurites, and bounded on the outer side 

 by the epimeron and on the inner side by the backward 

 growth of the endopleurite. These chambers may be 

 called the Pleural muscle chambers (Text fig. G, P.). 



There is also an inner chamber at each side, lying 

 between two consecutive endosternites, and separated from 

 one another by the median plate. We designate these the 

 Sternal muscle chambers (Text fig. 6, S.). 



These pleural and sternal muscle chambers contain 

 the muscles which w T ork the two basal segments of the 

 appendages in this region. 



The muscle chamber of the last walking leg is not 

 divided into parts owing to the absence of a separate 

 endopleurite in this somite. Therefore this last muscle 

 chamber may be known as the Pleuro- Sternal muscle 

 chamber (Text fig 6, PS.). Each of these chambers has 

 an antero-lateral prolongation, which extends forward as 

 far as the posterior face of the fourth thoracic arthro- 

 phragm. 



The fourth thoracic arthrophragm (Text fig. 5, .4.) 

 arises between the fourth and fifth thoracic somites. This 

 arthrophragm has already been described. 



