VII MOLLUSCA~THE BODY CAVITY 211 



XVIII. The Body Cavity. 

 Primaxy and Secondary Body Cavity, Pericardium, Pericardial Gland. 



The Mollusca are said to have a primary and a secondary body 

 cavity. The former is the system of lacunae and sinuses, into 

 which the arteries open, and out of which the veins, where these are 

 present, draw their blood. It has no epithelial walls of its own, its 

 boundaries are formed by connective, nerve, or muscle tissue, or by 

 epithelia, which, however, belong to other organs, such as the intestine, 

 the kidneys, or the body wall. 



The so-called secondary body cavity or coelom is, in most Mollusca, 

 very much reduced, usually consisting of only two small cavities, the 

 perieardium and the cavity of the gonads (testes, ovaries, or her- 

 maphrodite glands). The coelom is always lined by an epithelium of 

 its own, the coelomic epithelium, and corresponds with the true coelom 

 of the Annelida, which also possesses such an epithelium. Like the 

 latter, it is connected, by means of the nephridial funnel, with the 

 nephridia, which lead to the exterior, and in Molluscs are usually 

 found only in one pair. A probe can therefore be introduced through 

 the kidney into the coelom, i.e. into that part of it which, containing 

 the heart, is called the pericardium. The germinal layers must be 

 considered as proliferations of the coelomic endothelium. The epi- 

 thelium of the pericardium is, in very many Molluscs, differentiated 

 into glands, called the pericardial glands ; these probably may be 

 classed together with the kidney as excretory. 



We should be justified in assuming, a priori, that the lumen of the 

 genital glands of the Mollusca is part of a true coelom, and that 

 the germinal layers themselves, i.e. that complex of cells which yields 

 the eggs and spermatozoa, are outgrowths of the endothelial wall of 

 this coelom. Direct support is, however, given to this assumption by 

 the fact that in the Solenogastres, Sepia, and Nautilus, the sac of the 

 genital glands is in open communication with the rest of the coelom, 

 forming, in fact, an only partly distinct division of the same. 



In the Solenogastres {e.g. Froneomenia), the hermaphrodite gland lies above the 

 mid-gut as a long tube, which In transverse section appears heart- or kidney-shaped, 

 as its lower part bulges out on each side. Its shape is determined by the fact that 

 the mid-gut forms dorsally a narrow but deep furrow, which cuts into this glandular 

 tube from below. The tubular gland is divided into two lateral spaces by a partition, 

 whose endothelial wall is the place of formation of the eggs ; these lateral chambers 

 may again be traversed by septa, on which the genital products develop. This 

 division is especially distinct at the posterior part of the tube, the two chambers 

 being there completely isolated, and entering the pericardium separately as genital 

 ducts. 



If the secondary body cavity of Froneomenia is compared with that of an Annelid, 

 we iind the following differences : 



