HOMARUS AMERICANUS MILNE-EDWARDS 125 



anatomical and embryological study. Such is the case with the 

 cephalothorax of the lobster. Here, as we have seen, the somites 

 are fused together and the fused tergites form a dorsal shield 

 or carapace. The thoracic tergites have, moreover, produced 

 two lateral folds or branchiostegites for the protection of the 

 gills, these folds forming part of the carapace. Nothing shows 

 the limits of individual tergites and it is not even quite sure, 

 though very probable, that the transverse so-called cervical 

 groove corresponds to the division line between the sixth and 

 seventh somites, i. e., between head and thorax. The recogni- 

 tion of the sternites is somewhat easier, owing to the fact that 

 not all of them have fused so completely. The more difficult 

 among them, those of the second and third cephalic somite, 

 are described in the "Instructions." The endoskeleton is re- 

 stricted to the thorax and abdomen. In the thorax it forms 

 a complicated latticework connected with the sternites of the 

 exoskeleton and forming attachment points for muscles and 

 providing protection for the nervous system. We have already 

 seen that the endoskeleton arises as infoldings or apodemes of 

 the ectoderm between the segments of the body. Consequently 

 the endoskeleton itself is segmented, each segment consisting 

 of a pair of endosternites and a pair of endopleurites. The sternal 

 canal in which the nervous system is inclosed is formed by the 

 endosternites and their dorsal horizontal plates or mesophragmas. 

 The latter thus form the roof of the sternal canal, but this roof 

 is discontinuous, the canal being in reality a passage through 

 several arches. The paraphragmas or outer processes of the 

 endosternites connect the latter with the endopleurites. The 

 endoskeleton forms the peripheral hinges for the articulation of 

 the thoracic appendages in the shape of balls which are situated 

 on the epimeral plates. The median hinges, on the other hand, 

 are cup-shaped and are formed by the sternites. 



Appendages. Except for the first cephalic somite which 

 bears the compound eyes and the last abdominal or anal somite 

 called the telson, all the other somites have a pair of appendages. 



