MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 167 



The coelom is divided by an almost complete membranous partition into two chambers, the 

 ventral of which contains the heart and its auricles and the pericardial glands, and is therefore 

 called the pericardial chamber (Fig. 36), while the much larger dorsal and posterior chamber 

 contains the gonad and stomach and second loop of the intestine, and is called the genital chamber. 

 (Fig. 7.) 



The membrane separating the two divisions of the coelom was named by Huxley the pallio- 

 visceral ligament. (Fig. 36, p. v. 1.) It is attached postei-iorly to the ventral body wall between the 

 anterior and posterior ventral aponeurotic 1)ands. It extends forward from here in a nearly 

 horizontal plane, attached by its edges to the lateral walls of the body, and tinall3' is attached 

 anteriorly to the inner wall of the mantle fold, the anterior portion of the ligament forming the 

 ventral or posterior walls of the anterior renal sacs. 



When the pericardial chamber is opened the ventral body wall and the posterior portion of 

 the pallio-visceral ligament will be found to be very closel}' applied to each other. 



As a matter of fact the heart is within the pallio-visceral ligament, and not far from its 

 center. The heart projects from the under side of the ligament, while the branchial veins, 

 expanding near the heart to form the auricles, extend freely through the pericardial chamber to 

 the points where they enter the wall of the mantle. Necessarily the branchial veins are also 

 surrounded l)y extensions of the pallio-visceral ligament and covered liy the ccelomic epithelimii. 



It has already been intimated that the pallio-visceral ligament is not cjuite complete. There 

 are, in fact, three openings in it by which the pei'icardial and genital divisions of the ca?lom are 

 put into communication with each other. The smallest of these, and for a long time considered 

 to be the only one, lies just back of the left side of the heart. (Fig. 36, p. v-p. ap.) It is an ova! 

 opening, the longer axis of which is directed posteriorly, and when undisturbed the edges of the 

 opening lie against each other. The common septal artery passes through this opening into the 

 genital division of the ctelom. 



The other two openings through the pallio-visceral ligament lie at either side of and mostlj- 

 anterior to the heart (Figs. 36, 3S, and 39). These were lirst described by Huxley. Thej- 

 generally extend nearly to the middle line in front of the heart, leaving the heart suspended by 

 a narrow ligament anteriorly. But their size and position is subject to considerable variation. 

 In the specimen from which Fig. 36 was drawn the openings were widely separated anterior to 

 the heart, and the left opening was at least twice as large as the right. In Figs. 88 and 39 the 

 openings are seen to be much closer to each othei' in front of the heart, those shown in Fig. 39 

 being even less separated than those in Fig. 38. In Fig. 39, too, the left opening is very much 

 larger than the right, while in Fig. 38 the left opening is only slightly larger than the right. 

 The edges of both openings are attached to the dorsal and anterior side of the heart. 



The dorsal aorta passes through the left opening, and following the posterior side of the 

 hsemoccelic membrane for a short distance, penetrates this and runs forward in the hamoccel 

 above the oesophagus. (Fig. 7, Ao.) 



From the posterior wall of each renal sac glandular appendages hang into the pericardial 

 cavity. The secretory epithelium of these appendages is a portion of the coelomic epithelium. 



The genital division of the ccelom is much more extensive than the pericardial division. It 

 is traversed by several ligaments which support and inclose viscei'a. 



Principal of these is the genital ligament, within which the gonad is inclosed. This extends 

 from the posterior wall of tiie crelom, just above the origin of the siphuncle, to the pallio-visceral 

 ligament just back of the heart. (Figs. 7, 38, and 39.) At its upper end this ligament forms a 

 band a couple of centimeters in breadth, attached to the body wall obliquely, in a line directed 

 downward from the right to the left side. 



The gonad, being contained within this ligament, causes a great enlargement of its lower 

 portion, and finally projects into the pallio-visceral ligament, causing the latter to appear to be 

 attached to the ventral surface of the gonad. The posterior opening through the pallio-visceral 

 ligament is just to the left of this attachment, the ligament being continued on the right side of 

 the opening up over the surface of the gonad. In this way the couunon septal artery comes 



