EELATK^NS OF KIDNKYR IN HALIOTIS TTIBEROULATA. 98 



position in the sense of being further out of the chink 

 between foot and shell. 



{h) Place this ctenidinm at a higher level than the anus 

 and excretory openings, and thus assure it against damage 

 from the outgoing current. 



It may be urged against the suggestion — 



1. That, as Boutan says, we must not consider the excre- 

 tory, etc., organs in discussing the torsion, because in 

 ontogeny (in Acmtea) the torsion is completed before the 

 definite appearance of kidney rudiments. The torsion, how- 

 ever, entails such a serious disturbance of organs that its 

 appearance in ontogeny is peculiarly liable to be hastened, 

 for the earlier it appears the smaller is the derangement. 



2. That it proves too much ; in other words, that accord- 

 ing to it the pre-torsional left ctenidium should have dis- 

 appeared before the torsion was complete. This ctenidium 

 has disappeared in most Gastropods, but its occasional 

 persistence is not a serious difficulty. In the first place its 

 diminution would be retarded by the fact that its possessor 

 was adapting itself to a life on the shore, where the time 

 for bi^eathing dissolved oxygen would be limited, thus making 

 even a less efficient breathing organ temporarily valuable. 

 In the second place this ctenidium persists mainly in forms 

 which have evolved on special lines : 



(a) Among the Fissurellidae, where the deepening of the slit 

 and further changes have shortened the path of the outgoing 

 current, thus reducing the possibility of its interference with 

 the incoming one. 



{h) In the Haliotidte, which have certainly come off from a 

 very primitive prosobranch stock. Here, too, a secondary 

 downward tilting of the other side of the branchial cavity 

 has given this ctenidium an improved position. 



(c) In modern PleurotomariaB. The primitiveness of these 

 forms is well known, and their very deep-water habitat seems 

 likely to make respiration more difficult and so encourage 

 the retention of all available respiratory tissue. 



3. The other objection is that it accounts for little more 



