TOPOGEAPHICAL EELATIONS AMONG THE DOCOGLOSSA. 281 



It would be possible to describe tbe second stage of the process of consolidation 

 by saying tbat the posterior portion of the high visceral hump laid itself down on its left 

 side (PL 15. fig. 7). In such a process the fore-gut, whose front end was fixed, must have 

 undergone a counter-clockwise torsion approaching 90°. The torsion of the visceral 

 hump through 180°, to which all Gastropods have been subjected, had already had its 

 efi'ect on the fore-gut. The total twist on this organ in the Docoglossa therefore should 

 approach 270°. As Amaudrut, "Willcox, Robert, Woodward, and others have shown, 

 the cross-section of the gullet allows us to infer the amount of torsion to which it 

 has been subjected. This is possible because of the presence of a primitively dorsal 

 pair of longitudinal folds and a ventral subdivided fold opposite them. In the 

 Docoglossa these folds stretch very far back, and a series of sections allows us to trace 

 them round (PI. 17. figs. 22 & 23) till the dorsal pair, having passed the mid-ventral 

 line, mount far up the right wall. The maximum torsion traceable in Acmcsa (fig. 23) 

 seems to be about 250^ but it is as mvich as 300° in Patella. This correspondence with 

 theory is good evidence for the second stage of the consolidation-process above mentioned, 

 and further evidence is obtainable from the nervous system. 



In the Prostreptoneure, as in primitive Gastropods generally, the visceral loop of the 

 nervous system is in intimate relation with the fore-gut. As the latter moved to the 

 right, it must have pressed the visceral loop before it, and, since we find that loo]) on 

 the right side in all Docoglossa, this may be quoted as evidence of the second stage of 

 the consolidation. 



The extra 30° of torsion in Fatella and the separation of gut and visceral loop must 

 be dealt with later. 



Part VI. — Consolidation of the Visceral Mass. Later Stages. 



In AcmcBa virginea, testudinalis, corticata, and fraffilis the gut is arranged practically 

 as in Haller's types, but with two differences of detail (PL 16. fig. 11). The limb A is 

 placed as if it had been pushed back from the right anterior corner, and the arrange- 

 ment of the junction of JS and C points to the same inference. This is what is here 

 and above described as the third stage of the process of consolidation, a squeezing-in of 

 the viscera of the apical region in the right anterior corner — it is continued farther 

 in pursuance of the process of general concentration. At the same time, tlie viscera of this 

 region exerted a pressure on the fore-gut in the ventral region to the left, and thus gave 

 it a curve to the left {Q, fig. 11). The junction of regions B and C now formed a 

 loop L, which characterizes most of the remaining Docoglossa and which may be double 

 (fig. 11). Changes in the head-region carried this process of inpushing from the right 

 anterior corner still further, and may be referred to as the fourth stage of consolidation. 

 The food of the Docoglossa must be raked up from hard rocky or tough algal surfaces, 

 and in correlation with this the odontophore has grown relatively to the other parts and 

 differentiated in various ways so as to seciu'c firmness and exact symmetrical adjustment. 

 A further fact is that the buccal glands increased markedly in mass, numy forms 

 possessing two pairs. The acinous mass of the gland also shifted back from the wall 

 of the buccal cavity, thus increasing the free space above the radula, and accumulated 



