194 ELEMENTARY BIOLOGY. [CHAP. 



a. The sternum; its middle portion is laterally 

 compressed (cf. b.). It gives off (for each seg- 

 ment) two ingrowths or endosternites, which arise 

 close together, immediately internal to the bases 

 of the appendages. 



/3. The tergum; arched above, prolonged down on 

 either side to form the outer limb of the 

 branchiostegite. 



If examined carefully, there will be seen 

 arising from the cervical groove, close together 

 near the middle line, two small tubercles or 

 endotergites. They are most fully developed in 

 the Lobster. 



y. The epimera. Each is vertically elongated — 

 pushed up as it were upon itself — to form the 

 inner wall of a great chamber {branchial 

 chamber) lying under cover of the branchio- 

 stegite. From it the inner limb of the branchio- 

 stegite is derived. 



Note the endopleurites ; ingrowths of the op- 

 posite epimeral walls, which abut against the 

 upper and outer borders of the endosternites. 

 These, with the endosternites and endotergites 

 above mentioned, are, one and all, ingrowths 

 of the exoskeleton, developed in connection 

 with muscular attachment. Cf. Sect. C. 2. 



8. The branchiostegite; its outer limb is dense and 

 calcified, its inner one is very delicate, largely 

 chitinous and hirsute. 



3. The telson. 



Its whole tergal area is densely calcified, its sternal 

 one is largely chitinous. It bears, in both Lobster 



