120 ISABELLA M. DRUMMOND. 



place rapidly from this time. The tendency already seen in 

 tiie stomach to grow out into a third sac which follows the 

 coiling of the liver becomes considerably accentuated, while 

 the gonad grows very rapidly and soon passes right up to 

 the tip of the last coil. At the same time it loses its solid 

 character; and, becoming hollowed out, acquires an opening 

 into the duct, as described in the special part of this paper. 

 Meanwhile the mantle cavity deepens, and the rectum grows 

 forward to open near the anterior edge. It is during this 

 growth that it acquires the characteristic disposition of the 

 adult, passing from mid-dorsally behind, obliquely down- 

 wards and to the right, a disposition which is doubtless con- 

 nected with the sharp bend towards the right described in 

 Sta.ge n. The pericardium alone begins at this late stage to 

 show new relations, for it widens and becomes very irregular 

 in shape, spreading amongst the other organs of the body so 

 as to form a kind of general body-cavity. 



Monstrosities. — Whilst collecting material for the study 

 of the normal course of development a few monstrosities 

 were found which presented some remarkable features. 

 Although it seems impossible fully to understand the 

 meaning of all the abnormal conditions found in these 

 embryos, some of them seem to me to be of sufficient interest 

 to justify the insertion here of a description of the main 

 features of their organisation. 



1. The simplest of these abnormalities is a small embryo 

 between stages C and D in degree of development, perfectly 

 normal in every respect, but wholly reversed. The liver 

 and stomach form a bulge on the right side, while the mantle 

 cavity and rectum are on the left, and all the other organs 

 correspond in every particular. This is, so far as I know, 

 the first record of a normally dextral Prosobranch so 

 organised. It is unfortunately too young to show definitely 

 the manner of coiling. 



2. This embryo, shown in lig. M ii, is very remarkable. 

 A camera tracing was made while it was still alive, and the 

 organs put in partly from life and partly after preservation. 



