282 DR. H. J. FLETJRE ON THE EVOLUTIO:^ OF 



on the anterior surface of the visceral hump. These two causes further increased the 

 backward pressure {X). 



As region A of tlie gut moved backwards and inwards, the visceral loop of the 

 nervous system, moored probably by its osphradial and rectal nerves, remained on the 

 I'iglit and was in this way sejDarated from the gut. Its very reduced size among so 

 many of the Docoglossa is probably a consequence of this separation. The gut of the 

 Cyclobranch Ancistromesm, according to Haller's figure (PI. 15. fig. 10), shows the effect 

 of the first two stages of consolidation and of the others to some extent, but particiUarly of 

 a fifth stage. A fairly proximal part of region C, apparently situated previously on the 

 right side, has been pushed in over the dorsal surface of the visceral hump, or perhaps 

 rather has lengthened in this particular direction because of the direction of the pressure 

 on it. The pressure in question was due to the concentration of the shell-muscle on either 

 side, but it took effect maioly on the riglit ( Y), as the pericardium on the left was hardly 

 susceptible of further compression. The part of region C which has spread on the dorsal 

 surface of the visceral hump is referred to in later paragraphs as loop M; it characterizes the 

 group of the Cyclobranchs. The position of the loop C-D dorsal to B in Ancistromesus, 

 instead of at its left side in other types, is not a difference of theoretical importance. 



In Patella vulgata (PI. 16. figs. 12 & 13) are seen the efi"ects of all the processes thus 

 far euiunreated. 



Regions A and B and loop L are fundamentally as in Acmcea testudinalis, save 

 that A is pushed still farther back from the right anterior region, to which it is now 

 distinctly concave, forming the curve Qy. This is to be understood as a further eflfect of 

 pressure in processes 3-5. It is probable that this pressure has also increased the 

 torsion of the fore-gut, and so accounts for the extra 30° of torsion beyond the 270° 

 whose origin has already been traced. 



Processes 3 and 4 seem to have had the additional effect of pushing back a loop iV^ on the 

 ventral surface of the visceral mass, as usual from tlie right anterior corner. Process 6 

 accounts for the loop 31 on the dorsal surface of the mass, as in Ancistromestts. 



Patella ccerulea (PI. 16. figs. 14 & 16) is particularly interesting in that of itself it 

 makes us infer process 5. The junction of A and B and that of C and D are pushed in 

 from the right over the dorsal surface, and this process is also evidenced in the same 

 way l)y P. radians and P. ornata. In the two latter forms loop N is not apparently 

 develojoed. In Patina pellucida the gut resembles that of Patella ccRvulea, save that 

 the junctions mentioned are even more strongly pushed in over the dorsal surface, and 

 the loop Jf is for this reason pressed to the left ; loop iVis, however, not developed. 



In the Nacellidai we find very marked elongation of the gut, but it is arranged as in 

 Patina, save that loop M has its two hind limbs close to one another, and they run round 

 the left side to the back in close contact with B. The inpushing from the riglit is very 

 marked, and may account for this change, which, however, may be due to a close 

 connection between Jfand B or Jf and the loops C-B at an early stage. At aU events, 

 there is no fundamental difference between Nacella and Patina in this respect. 



In Scurria the Docoglossan gut attains its greatest length, but even here it only 

 shows local lengthening of what is fundamentally a Nacellid gut. 



