TOPOGEAPUICAL EELATIOXS AMONG THE DOCOGLOSSA. 



283 



"We therefore see that the Docoglossaa gut is arranged on what is fundamentally 

 a single type complicated by progressive Icugtliening in a confined space. 



From the disposition of the parts of the gut and of the various organs of the hump it 

 has thus been possible to infer the course of the process of consolidation of the visceral 

 hump. This has been described as consisting of five stages, which, though necessarily 

 separately discussed, must be understood to have acted in part simultaneously. 



Part VII. — Summary of Docoglossan Evolution. 



Other processes enumerated contributed to the development of the Docoglossan 

 characters, so that the evolution of the group may be summarized as follows : — 



I. The foot shortened and broadened into its characteristic oval shape, and the tail- 



process disappeared. This caused a forward pressure on the viscera, which is 

 discussed in dealing with the third stage of the consolidation-process (IV. 3, below). 



II. The shell-edge grew downwards and outwards, giving the cone-like form. Growth 



was particularly marked along the posterior edge, and in this way the apex came 

 to lie far forwards. Correlated with this are the reduction of the epipodium, the 

 sensory differentiation of the mantle-edge, the degeneration of the eyes, and the 

 evolution of the horseshoe shell-muscle. 

 III. The changes in II. reduced the efficiency of the branchial cavity, which was 

 already impaired by the loss of the slit. As a result, we find alteration of the 

 position of the Monobranch ctenidium, and complete disaj)pearance of the 

 ctenidia io Cyclobranchs. In both groups the mantle-skirt becomes an important 

 respiratory organ, and in the latter group a circle of pallial gills is developed. 

 Concomitant changes occurred in the heart, which moved to the left side. 

 IV. Consolidation of the viscera. This has been described in five stages : — 



1. The contents of the spiral settled down in the space above the broadened foot. 



The viscera of the base of the spire went from front to back ventrally, and 

 those of the apical region from the back to a right anterior position dorsally. 



2. The viscera farther consolidated so as to reduce height. The upper parts of 



the mass, together with the rectum, drew to the left as the pericardium (III) 

 moved thither, and thus came to lie along a line going obliquely backwards 

 from the right anterior corner to the left side. The lower viscera, on the 

 other baud, arranged themselves along a line going forwards from the right 

 side to the head. They crossed under the ujiper viscera about beneath the 

 apex of the cone. 



3. The shortening of the antero-posterior axis caused pressure forwards on the 



viscera and pericardium. The apical viscera were at the same time squeezed 

 in under the branchial cavity, and exercised a pressure backwards from the 

 right anterior corner. 



4. The growth of the buccal mass and the increase of the salivary glands increased 



tlie backward pressure just mentioned. 



5. The concentration of the shell-muscle led to inpushing of the viscera, and the 



eflfects are visible particularly on the right side. 



y<^''^^ 



<E i^ 



