270 THE ANATOMY OF INVERTEBRATED ANIMALS. 



however, that these cells are very incomplete, communicating 

 with one another anteriorly and posteriorly by the large 

 apertures left between the endosternites and endopleurites ; 

 and laterally, by the spaces between the endosternites, 

 through which each series opens into the sternal canal ; while 

 above, they are in free communication with the thoracic 

 cavity. The apodemes give attachment to the muscles of 

 the api3endages, while the chain of ganglia and the sternal 

 artery lie in the sternal canal. 



The appendages of the penultimate resemble those of 

 the last thoracic somite, but the three preceding pairs differ 

 from them by being chelate — that is, by having the posterior 

 distal angle of the propodite produced so as to equal the 

 dactylopodite in length, and thus constitute a sort of oppos- 

 able finger for it (Fig. 71, G-, 6, 7). The first ambulatory or 

 prehensile limb, again, is remarkable for its great size and 

 strength, and for the ankylosis of its basipodite with the 

 ischiopodite. 



The four anterior pairs of ambulatory limbs differ from 

 the last pair in possessing a long curved appendage (Fig. 71, 

 iV), which ascends from the coxopodite, with which it is artic- 

 ulated, and passes into the branchial chamber, in which it lies. 

 This is the eplpodite ; its relation to the function of respira- 

 tion will be adverted to presently. 



The sterna, which are wide in the three hindmost thoracic 

 somites, become very narrow and almost linear in the ante- 

 rior ones. They and their apodemes, however, remain per- 

 fectly recognizable. 



The sternal regions of the three maxillipedary somites 

 have the same characters, their appendages and articular cavi- 

 ties becoming smaller ; while, by the contemporaneous exces- 

 sive narrowing of the interarticular regions of the sterna, 

 these cavities are closely approximated. 



The sternum of the next anterior somite (bearing the 

 seconl pair of maxilhe), on the other hand, though very nar- 

 row from before backward, has a considerable width, and its 

 articular cavities, already much larger thnn those of the ante- 

 rior maxillipedary somites, are consequently thrown outward. 

 Hence results a sudden widening of the second maxillary, as 

 compared with the fiirst maxillipedary somite ; and, as a con- 

 sequence, we find a deep fold or depression on the sides of 

 the body where these two somites join. This fold is directed 

 upward and backward on the flanks of the body, parallel with 

 an important impression on the carapace, the cervical groove. 



