284 DR- H. J. FLEUEE ON THE EVOLUTION OF 



Part VIII. — Appendix to Evoltjtionakt Sketch. 



It now remains to deal with various details of the group's evolution that would have 

 appeared to be digressions had they been treated in the discussion of the main scheme. 



The Buccal Mass has been specialized in important points, whose bionomical correlatives 

 must be briefly mentioned, though a detailed discussion would not be profitable without 

 comparison throiighout witli other primitive Gastropods. The specialization is on different 

 lines in Acmcea and Patella, thus supporting the view that Cyclobranchs and Mono- 

 branchs have diverged from a common ancestor rather than evolved one from the 

 other. 



The odontopliore of the Prostreptoneure showed in transverse section a V-form, which 

 was modified among the Docoglossa as an adaptation to efficiency of raking. The 

 cartilage-pieces drew close together and altered so as to fill the groove, the remains of 

 whicli are more conspicuous in Acmcea than in Patella. The teeth of the radula have 

 become strong and specialized hooks or claws, and the differentiated median tooth has 

 been reduced or has become similar to the others, as a specialized median piece would 

 scatter rather than rake in. 



In species of Acmcea, which in several cases feed on calcareous Algae, the cartilages of 

 either side have fused, giving a much enhanced firmness. 



Among the Patellidse, on the other hand, we find general growth, increase of the 

 number of muscles, and differentiation of extra cartilages to which muscles are attached, 

 and which thus increase the possibilities of adjustment. 



Reference has already been made to the jaws, the buccal glands, and the lost spiral 

 CEecum of the stomach. It is noticeable that the oesophageal pouches of the Docoglossa 

 do not show the marked glandular development which characterizes these organs in 

 Raliotis, &c., and, perhaps in correlation with this, the valves are reduced which in the 

 latter prevent food from returning to the buccal cavity when it is in these pouches. 

 The oesophagus or crop has become very much more complex within the group. Withia 

 the limits of the Docoglossa the nervous system has not greatly altered. The changes 

 in the visceral loop have already been mentioned. The anterior ends of the pedal 

 commissures are far wider apart in Acmseidae and Nacellidse than in Patellidae. The 

 latter show therefore a concentration perhaps correlated with the increased efficiency of 

 adhesion which undoubtedly characterizes them. The figures of the Docoglossan 

 nervous systems due to Bouvier, Pelseneer, Haller, and Miss Willcox are so widely distri- 

 buted in test-books that I have thought their reproduction in connection with this 

 paper unnecessary, 



Thiele has observed various special patches of sensory epithelium whose phylogenetic 

 importance is not yet determined, so I pass them by and give merely a reference to 

 pages 326 and 327 of his recent paper (46). 



The necessity for compactness of the viscera explains the development by the 

 Docoglossan right kidney of superficial lobes around the visceral hump superseding the 

 intervisceral lobes of the kidney of the ancestral form. The only intervisceral lobe 

 which remains is the small subrectal one into which the reno-pericardial canal opens. 



