GASTROPODA 375 



of the mantle-cavity (pulmonary cavity) much restricted : in the Oncliidiidae 

 the lung sac is said to be absent, but this is doubtful (Bergh. Morph. Jakrb. 10). 

 A few of the Pulmonata have partly or wholly reverted to an aquatic life 

 (the so-called fresh-water Pulmonata) ; some of these forms (Limnaeidae) are 

 really air-breathers, and make periodic visits to the surface to procure air, 

 while others (Amphibola, Siphonaria, Gadini/a, Ancylus, Limnaeidae of deep 

 lakes) are truly aquatic, as the lung sac is always filled with water. 



Body-cavity. In Gastropoda there is usually a well-developed 

 perivisceral cavity in relation with the alimentary canal or with 

 the anterior part of it. It is the cavity which is opened up in 

 dissecting the alimentary canal of Chitonidae^ and the anterior part 

 at least of the alimentary canal of most other Gastropoda. We 

 may call this cavity the perivisceral. There is also another cavity, 

 which has no connection with the perivisceral, and is called the 

 pericardial because it is related to the heart. Whereas it is quite 

 certain that the pericardial cavity is a part of the coelom, the 

 nature of the perivisceral cavity is doubtful. By most anatomists 

 it is regarded as haemocoelic in nature, and this is probably the 

 correct view, but recently it has been interpreted as a part of the 

 coelom at least in the Chitonidae. This view is not, however, 

 supported by any evidence anatomical or embryological, for the 

 space in question has no connection with the nephridia or gener- 

 ative organs, and has not been traced from the paired cavity, 

 which appears at an early stage in the development of Chiton 

 and is probably coelomic. We may, therefore, safely say that so 

 far as our knowledge at present goes the perivisceral cavity of 

 Gastropoda is a part of the vascular system, and therefore 

 haemocoelic. 



It has been recently asserted* that in some of the Docoglossa the pericardium 

 has a considerable visceral extension. 



The coelom of the Gastropoda is in three sections : (1) the 

 pericardium ; (2) the nepliridia ; (3) the gonad. 



(1) The pericardium is in relation with the heart; it normally 

 communicates with the nephridial system, and part of its lining is 

 generally glandular and forms the pericardial gland. It has no 

 connection with the blood system. 



(2) The nephridial part of the coelom will be described below 

 under the head of excretory organs. 



(3) The gonad retains its connection with the rest of the coelom 

 only in the Aspidobranchia, in which it communicates with the right 

 kidney. In Chitonidae it is separated both from the nephridia and 



f B. Haller, " Studicniib. Docoglossc u. Rliipidoglosse Prosobranchicr," Leipzig, 

 1894. 



