ASTACUS FLUVIATILIS. 275 



pedary somites, the pleural, or free part of the carapace, 

 termed, from its function, the branehiostegite, or cover of the 

 gills, incloses a wide space, bounded internally by the epimera 

 of the somites. This is the branchial chamber. In front of 

 the maxillipedes and cervical fold, however, the chamber sud- 

 denly becomes narrowed by the rapid widening of the sterna 

 of the maxillary and mandibular somites, and by the lowering 

 of the point at which the reflection of their epimera into their 

 pleura takes place. Finally, on the antennary somite, and in 

 front of it, the pleuron becomes a mere fold separated by a 

 shallow groove, the representative of the branchial chamber, 

 from the epimera. 



On the dorsal surface there is no indication of an} 7 divis- 

 ion cf the carapace into terga corresponding with the sterna 

 of the somites, but it is marked by a well-defined, curved 

 groove, the posterior convexity of which extends across the 

 carapace, rather behind its middle, and the lateral portion of 

 which runs downward and forward, toward the anterior part 

 of the antennary sternum. This is the cervical groove / that 

 part of the carapace which lies in front of it is the cephaloste- 

 gite, while that which is behind is the omostegite. 



The omostegite, again, is divided into three portions by a 

 groove on each side of the middle line — the branchiocardiac 

 grooves. The branchiocardiac groove, and the lateral por- 

 tion of the cervical groove, on the dorsum of the carapace, 

 correspond very closely with the line at which the epimeral 

 is reflected into the pleural membrane, on its ventral surface. 

 The transverse portion of the cervical groove, on the other 

 hand, corresponds with the posterior boundary of the stom- 

 ach and the anterior extremity of the heart, and continues 

 inw^ard the line of the cervical fold ; so that, in a longitudi- 

 nal section of an A.stacus, the direction of the cervical fold, 

 if followed upward and backward, strikes against the inner 

 surface of the carapace, at a point corresponding with the 

 summit of the cervical groove, on its outer surface. By cut- 

 ting through the cervical fold, therefore, through the mem- 

 brane joining the second maxillary with the first maxillipe- 

 dary sternum, and through the carapace in the transverse 

 part of the cervical groove, it is possible to separate an ante- 

 rior portion of the cephalo-thorax, containing the whole of 

 the cephalostegite, and the first six somites, with their ap- 

 pendages, from a posterior portion, consisting of the omos- 

 tegite, and the last eight cephalo-thoracic somites. And, in 

 making this artificial separation, we should be merely carry- 



