458 SANGUIFEROUS SYSTEM. 



and they communicate by a single valved orifice with the 

 thick- walled muscular cavity of the ventricle. In the oyster 

 (120. A.) where the body is much compressed and very 

 narrow transversely, the two parts of the auricle (120. A./.) 

 are united together on the median plain and constitute a 

 single cavity, communicating however, by two distinct ori- 

 fices with the ventricle (120. A, g}. The auricle and ventri- 

 cle are inclosed in a capacious pericardial cavity, which is 

 situated, in the oyster, in the concavity of the great adductor 

 muscle, (120. A. /.) The ventricle is generally single and 

 placed in the middle of the back, as seen in the spondylus 

 (120. B. *.) receiving on both sides, from the divisions of the 

 auricle, (120. B. h.} the arterialised blood transmitted by the 

 palleal, (120. B./.) and the branchial (120. B.#.) veins. As 

 the rectal portion of the intestine (120. B. i. e.) occupies the 

 median line of the back, it commonly perforates the ventricle 

 in its course to the anus, and thus indicates a partial internal 

 division of this muscular cavity. In some, as the teredo, 

 the ventricle is cleft posteriorly into two parts, but commu- 

 nicates by a single opening anteriorly with the aorta ; and in 

 the area noa, (Fig. 134) the ventricle is entirely divided into 

 two distinct and separate parts, (134. d. d.) each of which 

 communicates by a distinct bulbus arteriosus ( 134. e. e.) with 

 the great trunk of the anterior (134. /.) and posterior (134. 

 g.) aorta. The auriculo-ventricular and aortal orifices of the 

 ventricle are furnished with semilunar valves which prevent 

 the return of the blood, and the bulbus arteriosus is most 

 apparent at the origin of the aorta where, as in the area noce 

 and the ostrea, the ventricle is not perforated by the intes- 

 tine. The ventricle, as shown by Cuvier, is divided into two 

 separate parts, also in the lingula^ among the brachiopodous 

 conchifera, and both the anterior and posterior aortse, as well 

 as the ventricle, are frequently traversed by the intestine in 

 the bivalved mollusca. 



When the ventricle is median and perforated by the intes- 

 tine, it is commonly elongated and fusiform, and tapers 

 anteriorly and posteriorly into the commencements of the 

 corresponding aortae. The anterior aorta is mostly a visceral 

 artery, supplying the stomach, the liver, the ovaria, and 

 other abdominal organs, and the posterior aorta has ge- 

 nerally a musculo-cutaneous or a palleal distribution. In 



