112 ISABELLA M. DRUMMOND. 



characteristic creeping sole. The visceral hump is much 

 developed and surrounded by a strongly marked mantle fold. 

 Externally the most noticeable feature is a prominent bulge 

 on the left side, which appears almost like the first formation 

 of a coil (fig. 12). A transverse section across the visceral 

 hump, shows, however, that it is simply a bulging out of the 

 side of the body where the liver and stomach are located. A 

 comparison of figs. 11 and 12, in fact, makes clear that this 

 prominence on the left side corresponds to the original apex 

 of the visceral hump, which has become still further tilted in 

 the same manner as before, thus leadiug to a rearrangement 

 of the organs when orientated with reference to the head and 

 foot, while their mutual relations are retained. The liver 

 now lies ventrally, as well as slightly to the left of the 

 stomach. 



Both liver and stomach are entirely on the animal's left, 

 and the mantle cavity lies entirely on the right, while the 

 anus, also, is displaced from the middle line and has travelled 

 towards the right. Though readily seen by a comparison of 

 transverse sections, this tilting is not so obvious when the 

 embryo is examined entire, as it was at an earlier stage. 

 This seems in part due to the great growth of the whole 

 posterior region of the visceral hump, which has caused the 

 rectuDi to bend forward to the anus, and the mantle cavity 

 to take up such a position that the two original depressions lie 

 almost vertically one above the other instead of almost horizon- 

 tally, as in fig. 1 1, but chiefly is it the result of the rapid growth 

 of the mantle downwards on the right-hand side of the animal's 

 body, so that the right and left edges are now on about the 

 same horizontal level, and a kind of false external symmetry 

 is established in this respect. Rapid growth of the mantle 

 leads, of course, to rapid extension of the mantle cavity, and 

 consequently we find this far in advance of the preceding 

 stage. The two original depressions are now united below 

 the rectum, and form two horns which abut each against 

 the kidney of its respective side, and then join and widen 

 out to form the mantle cavity ; a slight extension of the 



