1894-95.] Dr Gilchrist on Torsion of the Molluscan Body. 361 
turned towards the right side, and the heart lies obliquely to the 
body, almost in the reverse direction to that in fig. 1. The most 
noteworthy change is, however, in the osphradium, both as to its 
structure and position. It has become quite a rudimentary organ, 
its function being probably taken up by a new set of organs alto- 
gether, viz., two patches of olfactory epithelium on either side of 
the head region (fig. 2, rhin.). We shall see that these new organs 
become more highly developed, and finally appear as well differen- 
tiated “ rhinophora,” with which the simpler form is probably 
homologous. Meanwhile the osphradium becomes completely rudi- 
mentary, and finally disappears altogether. It will be seen from 
the figure that the genital opening lies well within the pallial 
cavity, and that the anus occupies a posterior position ; the mantle 
itself is also of less extent than in fig. 1. 
The next form we take up is Acera (fig. 3). The foot here is 
much larger, and extends laterally in the form of distinct “ pleuro- 
podia.” The gill faces slightly more towards the posterior, but the 
heart does not point in the posterior direction so much as in Bulla. 
This is perhaps due to the fact that the pallial cavity may be firmly 
closed by a sphincter-like contraction, in which the anterior limb of 
the gill attachment is drawn away from the heart and sidewards. 
This mechanism may be observed in the living animal, the slightest 
Fig. 2. 
Fig. 3. 
