78 THE GASTROPODA 
tion. This tendency to detorsion may be observed in exceptional 
cases among the Streptoneura (Pterotrachea, Fig. 143), but it is 
specially characteristic of the whole group of the Euthyneura, lead- 
ing to the untwisting of the visceral commissure, which, in this 
group, is obviously twisted only in Actaeon (Fig. 57). When detorsion 
is carried to its extreme limit as in Pterotrachea, it is accompanied 
by a reduction or disappearance of the mantle and visceral sac and 
opisthobranchialism. In the least specialised Opisthobranchs and 
Pulmonates the detorison is not complete, and the pallial aperture is 
carried only to the right side (Figs. 148,67); but inthe most specialised 
Fic. 58. Fic. 59. 
Philine aperta, ventral aspect. a, anus ; Oncidiella patelloides, ventral aspect. an, 
f, foot; g, gill; gl.f, glandular fossa; g.0o, anus; gl, tentacular gland; 0, mouth; o.f, 
genital orifice (seen through the foot); k.o, female orifice ; 0.m, male orifice; p, foot; pa, 
renal pore; os, osphradium; pa, inferior mantle ; pns, pulmonary orifice; si.p, lateral 
pallial lobe. (After Guiart.) groove ; te, tentacle. 
forms the anus and the pallial cavity (if the latter is retained) are 
moved back to the posterior extremity of the body, as in Philine 
(Fig. 58), Aplysia (Fig. 154), Doridomorpha (Fig. 79), and many other 
Nudibranchs, such as Janus, Alderia, Limapontia, and Cenia; and 
among Pulmonates in Testacella, Vaginula (Fig. 179), and Oneediwm 
(Fig. 59). In this manner a secondary external symmetry is re- 
established. The detorsion of the organism is complete in the 
Tectibranch Cavoliniidae (“straight Thecosomatous Pteropods”), in 
which one may recognise a torsion of 180° in a direction opposite 
and equal to that of the original torsion, the result of which is that 
the genital duct is twisted round the alimentary canal and the 
pallial cavity is shifted to the ventral surface (Fig. 60). It should 
