554 THE INVERTEBRATA 



some forms (Bullomorpha (Fig. 387 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 

 untwisted {Aplysia (Fig. 388 A)). The Opisthobranchiata, it is plainly 

 seen, are derived from the Monotocardia amongst the Streptoneura, 

 since they have only a single ctenidium, a single auricle and a single 

 kidney. They have not attained to complete bilateral symmetry, be- 

 cause the mantle cavity is still on the right side where yet present 

 (tectibranchs), 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. 388 C) their descent 

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

 visceral hump which undergoes torsion (which reverses later). The 

 adult shows evidence of streptoneurous ancestry in the presence of 

 the anus at the right-hand side. In Doris (Fig. 388 B) the anus and 

 renal aperture are median, but the genital aperture is still situated on 

 the right side. 



The last division of the Gasteropoda is the Pulmonata, which is 

 usually united with the Opisthobranchiata to form the group Euthy- 

 neura. But "euthyneury " or symmetry of the nervous system (more 

 particularly the "visceral" part of it) is arrived at in different ways 

 in the two divisions. In the Opisthobranchiata, as shown above, it is 

 by detorsion. In the Pulmonata, however, the shell is retained and 

 the visceral hump coiled in typical members of the group (land snails). 

 But the visceral loop is shortened and untwisted at the same time 

 (Fig. 387 A, B), and finally it is incorporated with its ganglia into the 

 circumoesophageal nerve collar, so that the nervous system becomes 

 symmetrical. The most primitive members of the Pulmonata still show 

 a twisted visceral loop which is beginning to shorten. All the group 

 have lost the ctenidium but they retain the single auricle which shows 

 them to be derived from the Monotocardia. This was brought about 

 by a chain of circumstances involving migration from sea to shore. 



The type of the Gasteropoda which is usually given for dissection 

 is Helix (either H. aspersa, the common English garden snail, or 

 H. pomatia, the edible snail). It possesses many features which are 

 common to the whole of the Gasteropoda, but as has been seen above, 



