18 SCIENCE PROGEKESS. 
anal papilla represents an approximated posterior pair of 
branchial sense-organs, since the anterior osphradium and 
both gill-plumes are all innervated from the outer visceral 
nerve. 
Euthyneurisnt.—Since the publication of Spengel’s paper 
on the olfactory organ and nervous system of Mollusca, a 
division of the Gastropoda into two groups, the Strep- 
toneura and the Euthyneura, has been generally adopted. 
This classification has been accepted, moreover, not merely 
as an expression of the anatomical facts concerning the 
condition of the visceral loop in the two groups, but as a 
classification of phylogenetic significance. It is to be in- 
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 Acton 
(= Tornatella), however, have completely altered the posi- 
tion of affairs. Acteon 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 Acteon 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 che 
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 
