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 Adaeon (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 in the most specialised 



Phillne aperta, ventral aspect, a, anus ; 

 /, foot; g, gill ; glj, glandular fossa; g.o, 

 genital orifice (seen through the foot) ; k.o, 

 renal pore ; os, osphradium ; pa, inferior 

 pallial lobe. (After Quiart.) 



Oncidiclla patelloideg, ventral aspect, an, 

 anus ; gl, tentacular gland ; o, mouth ; o./, 

 female orifice ; o.m, male orifice ; p, foot ; pa. 

 mantle ; pns, pulmonary orifice ; ti.p, lateral 

 groove ; te, tentacle. 



forms the anus and the pallial cavity (if the latter is retained) arc 

 moved back to the posterior extremity of the body, as in Philine 

 (Fig. 58), Aplysia (Fig. 1 54), Doridomorpha (Fig. 79), and many other 

 Nudibranchs, such as Janus, Alderia, Limapontia, and Cenia ; and 

 among Pulmonates in Testacella, Vaginula (Fig. 179), and Oncidium 

 (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 tiie alimentary canal and the 

 pallial cavity is shifted to the ventral surface (Fig. 60). It should 



