GASTEROPODA 



479 



of gasteropods called the Opisthobranchiata which show that the 

 changes occurring in torsion are to a certain extent reversible. They 

 have the ctenidium pointing backwards, the auricle behind the 

 ventricle and the visceral loop untwisted and symmetrical. There are 

 some forms (BuUomorpha (Fig. 363 C, D)) included in the Opistho- 

 branchiata which possess a complete coiled shell, but show only 90° 

 of torsion, so that the anus and the ctenidium point laterally instead 

 of anteriorly. The visceral loop also shows untwisting and the forms 

 in this division are thus supposed to show partial reversion of torsion 

 or detorsion. Forms like this pass into the typical opisthobranchs 

 with complete detorsion, in which the shell is reduced or lost, the 

 ctenidium directed posteriorly and the visceral loop is completely 



Fig. 355. Diagram to illustrate torsion, when seen from above. A, Ancestral 

 gasteropod. B, 90° of torsion. C, Torsion completed (i8o°). After Naef. 

 an. anus ; au. auricle ; ce.g. cerebral ganglion ; M. mouth; ma.c. mantle cavity; 

 pa.g. parietal, ped.g. pedal, pl.g. pleural, vis.g. visceral ganglia. 



untwisted (Aplysia (Fig. 364 A)). The Opisthobranchiata, it is plainly 

 seen, are derived from the Monotocardia amongst the Streptoneura. 

 They have only a single ctenidium, a single auricle and a single kidney. 

 They have not attained to complete bilateral symmetry, because the 

 mantle cavity is still on the right side where yet present (tecti- 

 branchs), and the anus and genital aperture both open there. 



The disappearance of the shell and the consequent uncoiling of the 

 visceral hump, if not the cause of detorsion, is a constant accompani- 

 ment of the phenomenon. When it is complete, the mantle cavity and 

 even the ctenidium may disappear and we arrive at the group known 

 as the Nudibranchiata. In forms like Eolis (Fig. 364 C) their de- 

 scent is shown by the fact that they possess a veliger larva with a 



