8 MR MACDONALD ON THE ANATOMY 



produced by the consecutive action of the ciUa within the canal, proceeds in a 

 forward direction, though of course the current to which it gives rise takes an 

 opposite course. 



As compared with the bulk of the animal, the liver (Plate I. fig. 4, f ) is of small 

 size ; and its minute lobuli may be traced along the posterior wall of the short 

 intestine and stomach, and for some little distance farther, in front of the internal 

 organs of generation, which occupy the remainder of the visceral chamber. 



The Mantle, Respiratory and Circulatory Systems. — In Atlanta, Carinaria^ and 

 Cardiapoda the mantle permanently envelopes the viscera, and, keeping pace with 

 the development of these organs, furnishes the materials of growth to the protect- 

 ing shell. The course of the intestine, and the position of the heart and respi- 

 ratory surface, all of which constantly preserve a definite relationship to each 

 other, exhibit, with reference to the extremities of the body, the usual arrangement 

 prevailing amongst the so-called praso-branchiate gasteropoda, — that is, the ali- 

 mentary canal being bent upon itself, the rectum passes forwards, and the anus 

 terminates it anteriorly ; the branchiae also lie in advance of the heart, through 

 which the circulation proceeds in a backward direction. In Firoloides., on the 

 contrar}'', the mantle is everted and thrown backwards ; and the parts above 

 mentioned being more or less iutimatel}^ connected with it, suffer a remarkable 

 change of position, which will be better understood when we have considered the 

 anatomy of the organs contained in the visceral nucleus. Thus, the free border 

 of the mantle is, as it were, turned inside out, and so very much constricted as to 

 circumscribe a small oval opening (Plate I. fig. 4, n) on the right side of the visceral 

 mass, and immediately in front of the rectum. This opening is surrounded by a 

 little sphincter muscle, Avhich is intersected by numerous radiating fibres, so that 

 ample provision is made for its contraction and dilatation, frequently observed 

 with a degree of rhythmical precision in very recent specimens. 



I have never seen any branchial appendages, properly so called, in Firoloides^ 

 although one would imagine, had such been present, that they would be still 

 more apparent in consequence of the eversion of the mantle. Apart from the 

 idea that the general surface of the body may be more or less subservient to 

 respiration, the position of the auricle of the heart points out the locality in which 

 we might expect to find the organs especially adapted to this function. It must 

 be observed, however, that immediately behind the heart, and above the anal 

 lobes, a richly ciliated space or fossa (Plate I. fig. 4, q), with a prominent margin, is 

 constantly present ; and I am inclined to think that the little clusters of spherical 

 cells, represented in Plate I. fig. 4, r, as occurring upon the pallial wall of the 

 pericardium, may possibly be concerned in respiration, and in some way asso- 

 ciated with the modification of the mantle above explained. 



The heart, consisting of a single auricle and ventricle, rests upon the mantle- 

 chamber which lies between it and the rectum. The muscular fibres of the auricle 



