280 DE. H. J. FLEUEE ON THE EVOLUTION OF 



oblique in the, on the whole, more primitive Acmoea than in the more specialized Patella 

 (PI. 17. figs. 19 & 20). These pressures from behind, and from the right as the apical 

 viscera settled beneath the branchial cavity, may be referred to as the third stage of the 

 process of consolidation. 



Owing to the special circumstances of dearth of free space for the lengthening gut, 

 the topograjihical relations of the intestinal coils are of value in tracing out the history 

 of the consolidation. 



The gut of the Prostreptoneure has already been mentioned, and its probable disposition 

 was as follows : — 



1. It was essentially a U-tube with extra coiling on the intestinal limb of the U. The 



rectum ran through the pericardium (figs. 6 & 16). 



2. The parts of the gut were : 



{A) The Fore-gut making the proximal limb of the U. 



{B) The Stomach and the beginning of the intestine. This ran forwards, perhaps 



above A. 

 (C) The middle regions of the intestine which probably lay near £ proximally and 



near D distally. 

 (Z*) The terminal part of the intestine which came forwards through the peri- 

 cardium. 

 One of the first changes among the Docoglossa was the lengthening of the wide 

 stomach-region, ?". e. the first part of B, and it is permissible to suppose that this extension 

 would be dorsalwards where space was freer. It is permissible to correlate this 

 lengthening with the special difficulty of digesting the food, which consists mainly 

 of small tough Algae &c. growing on exposed rocks, or, in the case of many species of 

 Acmcea, of calcareovis coralline Algaj. 



After the first stage of consolidation of the viscera, it is therefore probable that B lay 

 more or less in the upper part of the visceral mass, A, C, and D as before. The loops 

 i?-Cand C-I) were certainly lengthening and the lengthening parts of B and Cand of 

 C and D would probably keep together. 



In the second stage of consolidation of the visceral mass, region A would go with the 

 lower part of the viscera, and would thereafter lie along a line going back from the 

 median anterior position towards the right. B, going with the upper viscera, would 

 make a dorsal loop from its junction with A round the back and left side and behind 

 the pericardium towards the right anterior corner. C, as before, would lie at first near 

 B and further on near I). JD would be pulled out towards the left as the pericardium 

 shifted in that direction ; it would therefore run back from the anus, behind the peri- 

 cardium to the left side, and would meet C at the back (see PI. 15. fig. 8). 



A gut practically of this type has been figured by Haller for Acmcea galathea (fig. 9) 

 and for Lottia viridida, and the preceding paragraphs on the disposition of the gut aim 

 at tracing its previous evolution. They arc admittedly hypothetical, but are inserted for 

 the sake of comj)leteness. Their correctness hardly affects the value of subsequent 

 deductions, as it would have been easy to take Acmcea galathea as our starting-point, 

 and derive the other types of gut from its actual arrangements. 



