54 SCIENCE PROGRESS. 



ferred that the two groups have been independently derived 

 from a common type of archi-Gastropod, possessing an un- 

 twisted visceral loop — the Prosobranchs (Streptoneura) by 

 the twisting of the loop, the Opisthobranchs and Pulmonates 

 (Euthyneura) by the mere shortening and concentration of 

 the untwisted loop. This view derives support from the 

 fact that the persistent ctenidium retains its primitive posi- 

 tion on the right side of the body in Opisthobranchs, while 

 in Prosobranchs it shows a marked displacement and lies 

 on the left side. Bouvier's observations on Actceon 

 ( = Tornatella), however, have completely altered the posi- 

 tion of affairs. Actceon is a very primitive Opisthobranch, 

 as may be inferred from the high development of its shell, 

 the persistence of its operculum, and the absence of pleuro- 

 podial fins. Bouvier tell us (4) that Actcson resembles the 

 Prosobranchs, not only in these points, but also in possess- 

 ing a distinct twist of the visceral loop (streptoneurism, 

 chiastoneurie). The ctenidium is innervated from a supra- 

 intestinal ganglion, which lies on the left side of the body. 

 We are accordingly led to the conclusion that the 

 euthyneurous condition of Opisthobranchs and Pulmonates 

 has not been directly inherited from the orthoneurous 

 ancestors of the Gastropoda, but has been derived from a 

 previously streptoneurous condition. In other words the 

 Opisthobranchs and Pulmonates have descended from 

 Prosobranch ancestors, and the right-sided position of the 

 gill-plume in Opisthobranchs is not primitive, but the result 

 of a secondary process of detorsion. 



Orthoneuroidism. — Without going further into the 

 matter it may also here be mentioned that the supra-in- 

 testinal commissure has been recently discovered in various 

 species of Nerita, Neritina, and Navicella by Boutan (2), 

 Bouvier (3^), and Haller (11) — a discovery which de- 

 stroys the last refuge of orthoneurism in Prosobranchiate 

 Gastropods. Streptoneurism may now be affirmed of all 

 Prosobranchiate Gastropods. 



Origin of the Moliuscan nervous system. — The attempts 

 of previous writers to explain the relations of the nervous 

 system of Mollusca have been based almost exclusively 



