274 DR. H. J. FLEUEE ON THE EVOLUTION OP 



Lrancliial cavity along the right side. The main argument there employed is that 

 the pretorsional right ctenidium and left kidney and gonad, the gonad probably earliest 

 of all, acquired predominance in their respective functions. This gonad is the important 

 one already in the Cephalopods, and its predominance would probably ensure a pre- 

 dominance of the kidney which it used as an excurrent channel. The gill of that side 

 would therefore be more liable to get soiled than its fellow, which latter would acquire 

 oreater importance. In such a condition, movement (that is, selection of variations of 

 position) of the branchial cavity up the right side would enhance the advantage of this 

 ctenidium, and therefore increase the efficiency of respiration. This had previously been 

 diminished through the development of the visceral hump above the posteriorly placed 

 branchial cavity. 



It seems to me that the paired shell-muscle is a post-torsional, or alnaost post-torsional, 

 development which may have helped to set the branchial cavity in a completely 

 forward position. Such a shell-muscle corresponds to a fairly symmetrical shell, and 

 the embryological and palseontological facts, though too scanty to be of the first 

 importance, seem to support the view here advanced, that the Prostreptoneure had not 

 advanced far in the direction of external asymmeti-y. There is also nothing in connection 

 with the torsion-process as just sketched out which in any adequate fashion accounts for 

 or involves the development of the typical asymmetric spiral. This last development 

 can be understood on other grounds, which will now be discussed, though it may 

 first be said that the difference between Thiele's opinion of the Prostreptoneure and that 

 advanced here is by no means fundamental, as a Haliotis not yet flattened would be very 

 much like PI. 15. fig. 3, in which, it is freely admitted, the amount of asymmetry may be 

 somewhat under-estimated. 



As the Prostreptoneure seems to have possessed an operculum, it must have been 

 able to protect itself from unfavourable circumstances by retracting the delicate parts 

 more or less into tlie shell. It is probable that variations of reduction of the left shell- 

 muscle accumulated very early among the Gastropods, in consequence of the increased 

 freedom this would give to the incoming stream bathing the more important ctenidium. 

 Disappearance of this muscle and inward motion of the area of insertion of its fellow 

 would facilitate complete retraction, there being now only one fixed point some distance 

 in, instead of two comparatively near the edge of the shell. With the loss of the 

 left shell-muscle, the left side of the shell and visceral mass lost their support, and we 

 may suppose them to have sagged, especially as a median spiral meant a centre of 

 o-ravity high above the foot and consequent unstable equilibrium, as the shell had by 

 this time grown considerably in length to increase the efficiency of retraction. Finally, 

 the shell has wound around its point of support, and so lodged a maximum length 

 of cavity in a minimum space and retained the centre of gravity as low as possible. 

 Such a course of evolution seems to correspond to the facts so far as they are known, 

 and obviates the necessity of connecting the extreme external asymmetry of Pleuro- 

 tomaria and Troclms with the torsion, and of ascribing it to the Prostreptoneure. The 

 conical spiral is therefore correlated with tiie creeping habit, and contrasts Avith the 

 symmetry of the Docoglossa adapted to an adhesive habit. 



