76 GOMPARATIVE ANATOMY chap. 



the pulmonary cavity, and at first forming a more or less closed channel along 

 which the urinary discharge can be forwarded from the base of the cavity to the 

 respiratory aperture. Some species of Bulimus and Helix (D). 



5. The secondary urinary duct becomes closed, and opens either alone or with 

 the anus into the pulmonary cavity. Some species of Buliimis, Eelix, Dmidebardia, 

 Vitrina, Hyalinia, Zonites, Arion, etc. (E). 



6. The end of the secondary urinary duct and the end of the rectum together 

 form a cloaca which is distinct from the pulmonary cavity," and opens close to 

 the respiratory aperture. Umax, Amalia, and some species of Davdebardia (F). 



When the primary urinary duct runs back along the kidney it is externally in- 

 distinguishable from the substance of the latter, and it thus often appears as if the 

 duct rose from the posterior end of the renal organ. 



The variations which occur in the position of the organs of the pallial complex 

 in the carnivorous Pulmonata are specially interesting. In a series of car- 

 nivorous forms, commencing probably with HyaUmia among the Stylommatophm-a, 

 and proceeding through Daudebardiu. to the extraordinary genus Testacella, we find 

 progressive diminution of the visceral dome and its displacement to the posterior 

 end of the body, simplification and diminution of the shell, and further, a shifting 

 back of the liver and genital organs from the visceral dome into the nuchal portion 

 of the ccelom, which now is found along the whole length of the dorsal surface of the 

 foot. Finally, in Testacrlla and certain Daudehardia, the visceral dome completely 

 disappears, and the pulmonary cavity covered by the shell is alone left, the cavity 

 reaching up to the apex of the shell. The floor of this cavity, and indeed the whole 

 cavity, with the mantle and the shell, sink down into the body. In this way 

 Testacella, which follows its prey, the earthworm, into its underground passages, is 

 admirably adapted to its manner of life ; its body is slender, and the somewhat 

 flat shell at its posterior end, which does not stand out above the surrounding sur- 

 face of the body, in no wa}' hinders its movements. These alterations, however, 

 especially the displacement of the visceral dome to the posterior end of the body, 

 are accompanied by important alterations of position in the pallial organs, which 

 finally lead to the condition called opisthopneumonic. 



It is important to note that concrescence of the mantle and the subjacent dorsal 

 integument is complete except at the respiratory aperture on the right, and that the 

 latter shifts farther and farther back, in its relation to the pulmonary cavity, till, in 

 Testacella, its position is almost terminal. 



The first important step in the displacement of the pallial organs is seen in 

 Daudelardia rufa. The pericardium, instead of lying far back at the base of the 

 pulmonarj' cavity, here lies far forward on its roof, so that by far the greater portion 

 of the vascularised pulmonary tissue lies on the roof behind the pericardium (Fig. 

 74 A). Daudehardia rufa is thus actually opisthopneumoniG. But in this case the 

 relative position of the ventricle and auricle is still unaltered. The auricle is, as 

 before, placed in front of the ventricle ; the pulmonary vein from the auricle is thus 

 obliged to bend round in order to run backward, while the aorta, which becomes 

 almost exclusively the anterior or cephalic artery, supplying that portion of the body 

 which lies in front of the visceral dome (by far the greatest part), must bend forward 

 from the ventricle. 



In another Daiidebardia, D. saulcyi, the case is somewhat similar, but the 

 kidney and pericardium together form a sort of sac which hangs down into the pul- 

 monary cavity from its roof. In this sac, the ureter lies dorsally and the peri- 

 cardium ventrally to the kidney. The floor of the cavity sinks right and left deep 

 into the subjacent region of the body. 



If we imagine that the pulmonary vein which runs back from the anteriorly 



