ASTACUS FLUVIATILIS. 



273 



side of its triangular anterior extremity it presents a wide 

 articular cavity for the articulation of the antennas. In these 

 organs (Fig. 72, .2?, d) the same parts can be recognized as in 



Fig. 72.— A. Anterior extremity of the cephalo- thorax of Astacus, with a portion of 

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

 thorax : a, rostrum ; b, ophthalmic peduncles ; c,antennulae ; d, antenna? ; <?. la- 

 brum ; /, metastoma ; g, oral aperture ; h, procephalie processes ; i, ophthalmic 

 sternum ; k, antennulary sternum ; /, 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 eoxocerite. Next, a basicerite, to the 

 outer portion of which a flattened plate, the representative 

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

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

 bearing a merocerite and carpocerite, while the last segment, 

 or procerite, 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 extremities two many-jointed 

 filaments, which probably represent the exo- and endopodites. 



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

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

 larger terminal podophthcdmite. 



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 



