ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 313 



7. Gasti'opoda. 



Relationships of the Gastropods. — Louis Boutan {Actes Soc. Linn. 

 Bordeaux, 1911), 71, 1-116, 25 figs.). There is ventral flexure in 

 Cephalopods and Scaphopods, and dorsal flexure in Pelecypods, but 

 nothing- comparable to these in Gastropods, where there is a rotation of 

 the anal region and of the top of the larval shell. Gastropods seem to 

 have a monophyletic origin. Prosobranchs and Opisthobranchs show a 

 close j)aralielism in their early stages and larvae. No Gastropod is twisted 

 on the longitudinal axis of the embryo in the cerebro-pedal region ; all 

 except the Amphineura are twisted on the longitudinal axis of the 

 embryo in the abdominal region. Only some are twisted on the longi- 

 tudinal axis of the embryo both in the abdominal region and in tlie 

 median region of the body. Those Opisthobranchs which have only a 

 feebly developed larval shell and have in the adult state a notseum do 

 not exhibit the torsion strictly so-called of the Prosobranchs. Those 

 Opisthobranchs which have a shell developed like that of Prosobranchs 

 and a cephalic disc slightly developed undergo true torsion, incompletely 

 at first and afterwards completely. They may then have an oesophagus 

 twisted upon itself and a Streptoneural nervous system as in Proso- 

 branchs. The archaic forms of Opisthobranchs should be looked for 

 among the Nudibranchs, some types of which show resemblances with 

 Amphineura. The resemblances which Opisthobranchs with a well- 

 developed adult shell present to Prosobranchs seem to be due to con- 

 vergence, probably due to the progressive disappearance of the not^um, 

 the progressive atrophy of the cephalic disc, and the formation of a more 

 encumbering shell. There is no detorsion in any Prosobranch or 

 Opisthobranch. Those, like the Nudibranchs, which are never twisted 

 in the larval stage in the median (oesophageal) region of the body 

 undergo no detorsion on becoming adults. Those, like some Tecti- 

 branchs, which are sUghtly twisted in the larval stage in the median 

 region of the body show a slight torsion in the adult state. Those, like 

 Actseon, which show complete torsion (torsion properly so-called) in the 

 median region of the body, and become Streptoneural in the larval state, 

 remain Streptoneural in the adult state. Finally, the Prosobranchs, 

 which all show torsion in the strict sense, retain this in adult life in spite 

 of all subsequent regularization of the body. The theory of detorsion 

 has rested on a confusion between strict torsion and the general rotation 

 of the anal region and the top of the larval shell. J. A. T. 



Breeding" and Habits of Periwinkle. — W. M. Tattersall {Sci. 

 Investigations Fisheries, Ireland, 1920, 1, 1-11, 1 pi). The supposed 

 eggs of Littorina littorea figured in Bronn's Tierreich, and often copied, 

 are those of L. ohtusata. The eggs of L. littorea, observed in an 

 aquarium, are pink, enclosed in transparent capsules like a soldier's tin 

 hat. The first ones usually contain a single ^gg, the later ones two or 

 more. There is no aggregation of capsules. The eggs are laid freely 

 on the shore and are in part borne about by the water. The same 

 female may go on depositing eggs intermittently for a month or more 

 (20th March to 24th April), the original act of copulation sufficing for 

 the whole, which is roughly estimated at 5,000. vSegmentation is 



