ASTACUS FLUVIATILIS. 273 



side of its triangular anterior extremity it presents a wide 

 articular cavity for the articulation of the antennae. In these 

 organs (Fig. 72, B^ d) the same parts can be recognized as in 



Fig. 72.—^. Anterior extremify of the cephalo-thorax of Astacus, with a portion of 

 the carapace removed. B. Vertical section of the anterior part of the cephalo- 

 fhorax : a, rostrum ; b, ophthalmic peduncles ; r,antei:nul8e ; d, antennae ; e. la- 

 brum ; f, metastoma ; g, oral aperture ; h. procephalic processes ; i, ophthalmic 

 sternum ; k, antennulary sternum ; I, antennary sternum or epistoma. 



the other appendages, viz., an imperfect basal joint, produced 

 into a prominent cone, perforated behind and internal to its 

 apex, and here called coxocerite. Next, a basicen'te, to the 

 outer portion of which a flattened plate, the representative 

 of the exopodite, and here called the seaphocerlte, is articu- 

 lated; while to its inner portion an ischiocerite is connected, 

 bearing a merocerlte and carpocerite, while the last segment, 

 ox procerlte^ consists of a long multi-articulate filament. 



The sterna of the next two somites are narrow and elon- 

 gated; that of the antennary somite is well calcified, but 

 that of the ophthalmic somite is almost entirely membranous. 



The antennules (Fig. 72, B^ c) present an enlarged trigonal 

 basal joint, succeeded by two others. These represent the 

 protopodite, and carry at their extiemities two many-jointed 

 filaments, which probablj" represent the exo- and endopodites. 



The peduncles of the eyes (Fig. 72, 6), lastly, are com- 

 posed of two joints, a small proximal basiojyhthalmite, and a 

 larger terminal podopTithalmite. 



Such are the structure and arrangement of the sternal por- 

 tions of the several cephalo-thoracic somites, and the nature 

 of their appendages. On regarding the sternal region as a 

 whole, there are yet some very important points (the morpho- 

 logical value of which has been fully pointed out by Milne- 

 Edwards) to be noticed. A longitudinal median section, 

 in fact, shows that, while a line drawn through the sterna of 

 the somites behind the mouth is nearly straight and parallel 



