532 H, p. KJEKSCHOW AGERSBORG 



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

 or by epithelia, which, however, belong to other organs, such 

 as the intestine, the kidneys, or the body-wall. The latter, 

 the so-called secondary liody-cavity or coelom, is in most 

 Mollusca very much reduced, usually consisting of only two 

 small cavities, the pericardium and the cavity of the gonads. 

 The coelom is always lined l)y an epithelium of its own, the 

 coelomic epithelium, and corresponds with the true coelom 

 of the Annelida, which also possesses such an epithelium. 



The primary body-cavity of Lang corresponds with the 

 perivisceral cavity of Sedgwick (1898: 375), who says : ' In 

 Gastropoda there is usually a well-developed perivisceral 

 cavity in relation with the alimentary canal or with the 

 anterior part of it.' The secondary body-cavity of Lang 

 corresponds to the pericardial cavity of Sedgwick. ' There is 

 also another cavity, which has no connexion with the peri- 

 visceral, and is called the pericardial because it is related to 

 the heart. By most anatomists the perivisceral is regarded as 

 haemocoolic in nature. It is part of the vascular system, and 

 therefore haemocoelic' 



Li regard to the T e t h y m e 1 i b i d a e Bergh (1 908 : 97) , 

 writing on M . rosea Rang, says : ' The cavity of the body 

 reaches to the region of the last of the papillae.' In M. 

 1 eon in a Gould, I find that the cavity extends anterior!}- 

 to the oesophagus, dorsally to the back, ventrally beyond the 

 genital ducts, posteriorly as far as the anus, back of which 

 the branching of the liver and the kidneys is so profuse, together 

 with the crossing of connective-tissue fibres, as to render it 

 very difficult to tell whether the perivisceral cavity extends 

 beyond the anus. This cavity is not a true coelom. It corre- 

 sponds with the primary body-cavity of Lang or the peri- 

 visceral cavity of Sedgwick. There is no definite termination 

 of an inner body-wall, although the muscle-wall seems to 

 represent one, but that is really superficial. Beyond the 

 muscle- wall the connective-tissue fibres run in all directions, 

 all through the cavity. It is, therefore, not a well-defined 

 cavitv. It is in this so-called cavity that all the visceral 



