ASTACUS FLUVIATILIS. 277 



ossicle. On each side a large, elongated postero-lateral or 

 zygocardiac ossicle (se), wider posteriorly than anteriorly, is 

 connected with the lower end of the antero-lateral ossicle 

 and, passing upward and backward, becomes continuous with 

 a transverse arcuated plate, calcified in its anterior moiety, 

 and situated in the roof of the anterior dilatation of the py- 

 loric portion; this is the pyloric ossicle (Fig. 73, py). 



These pieces, it will be observed, form a sort of six-sided 

 frame, the anterior and lateral angles of which are formed by 

 movable joints, while the posterior angles are united by the 

 elastic pyloric plate. 



From the middle of the cardiac piece a strong calcified 

 urocardiac process (ca r ) extends backward and downward, 

 and, immediately under the anterior half of the pyloric ossi- 

 cle, terminates in a broad, thickened extremity, which presents 

 inferiorly two strong rounded tuberosities, or cardiac teeth. 

 With this process is articulated, posteriorly, a broad pre- 

 pyloric ossicle, which passes obliquely upward and forward, in 

 the front wall of the anterior dilatation of the pyloric portion, 

 and articulates with the anterior edge of the pyloric ossicle, 

 thus forming a kind of elastic diagonal brace between the 

 urocardiac process (ca') and the pyloric ossicle. The inferior 

 end of this pre-pyloric ossicle is produced downward into a 

 strong bifid urocardiac tooth (ac). Finally, the inner edges 

 of the postero-lateral ossicles are flanged inward horizontally, 

 and, becoming greatly thickened and ridged, form the large 

 lateral cardiac teeth (cc). The membrane of the stomach is 

 continued from the edges of the pre-pyloric to those of the 

 postero-lateral ossicle in such a manner as to form a kind of 

 pouch with elastic sides, which act, to a certain extent, as a 

 spring, tending to approximate the inferior face of the pre- 

 pyloric ossicle to the superior face of the median process of 

 the cardiac ossicle. 



The result is, that there is a certain position of equilibrium 

 of the whole apparatus, when the urocardiac process and the 

 pre-pyloric ossicle make a small angle with one another, and 

 the antero-lateral ossicles form an almost unbroken transverse 

 curve with the cardiac. When undisturbed, the apparatus 

 tends to assume this position. 



Two pairs of powerful muscles are attached to this gastric 

 skeleton. The anterior pair arise from the procephalic pro- 

 cesses, and are inserted into the roof of the stomach, some- 

 what in front of the cardiac ossicle ; the posterior have their 

 origin in the carapace immediately above and behind the 



