TOPOGEAPHICAL EELATIONS AMOJsG THE DOCOGLOSSA. 279 



The cliange from ctenidial respiration to respiration by the pallial gills and the roof of 

 the branchial cavity has led to dwindling of the efferent ctenidial vein and strengthening 

 of the pallial vein which joins it (PI. 16. figs. 16 & 17). Small vessels from the roof 

 of the branchial cavity have become connected with the auricle direct (tig. 17), instead 

 of with the efferent ctenidial or the pallial vein. 



Part V. — Consolidation of the Visceral Mass. First and Second Stages. 



With regard to the consolidation of the viscera, it has been argued in preceding 

 paragraphs : — 



1. That the contents of the ancestral spiral ■visceral hump settled in the space left free 



between the broadening foot and the conical shell, and that the contents of the 

 upper part of the spiral were laid down with apex forwards over those of the 

 basal part (PI. 15. fig. 4). 



2. That the' apical viscera settled to the right, as the left side was occuj)ied by the 



ctenidium, which was still important. 



3. That the pericardium moved to the left side after the disaj)pearance of the right 



ctenidium, and pulled the rectum with it for some distance. 



4. That the anus and excretory openings moved to the right to the space left free by 



the disappearance of the right ctenidium. 



In this way the rectum became connected with the upper portion of the viscera and 

 comiected them with the pericardial region. 



The steps in the process of consolidation of the visceral mass ai'e inferred largely from 

 a study of the topography of the included organs. For the sake of brevity and clearness, 

 however, the stages of consolidation will first be described, and then will follow an 

 account of their effects on the mutual relations of the different organs. 



The reduction of height was an important consideration for the Docoglossa, and this 

 led to the settling of the high visceral mass with its " upper " and " lower " parts side 

 by side. The apical viscera lay to the right, and the remainder of the upper region was 

 affected by the shift of the pericardium to the left, so that it followed and came to lie 

 along a line going obliquely backward from right to left (PI. 15. fig. 7). The lower 

 portion therefore settled on the right side in the posterior region, and led forwards to 

 the left beneath the pericardium and so on to the median anterior head. The lower 

 viscera thus crossed beneath the upper just about beneath the last remains of the spiral, 

 and it is possible that the coincidence of position is evidence of correlation between these 

 changes. This stage is represented in fig. 7, and may be called the second stage of the 

 consolidation of the viscera. 



The general concentration made the apical viscera squeeze under the branchial cavity, 

 and this pushed the lower viscera in this plane towards the left (curve Q). A special 

 pressure Avas also exerted from behind {Z, fig. 7) as the antero-])Ostcrior axis shortenetl, 

 and this pressure helped to reduce the branchial and pericardial cavities. It is interesting 

 to note that Haller found a more spacious pericardium in Acmcea galathea, which seems 

 to be very primitive, than in other forms. Another effect was that the pericardium was 

 pushed forwards, and we notice that the posterior boundary of the pericardium is more 



39* 



