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.— 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 ; <°, antecnulae ; d, antennae ; e. la- 

 brum ; /, 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 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 prooerite, 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 podophthalmite. 



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 



