62 NATURAL SCIENCE. July. 



number of Streptoneura, and a Nudibranch Euthyneur, Bathydoris), 

 and of the pallial eyes in the Pectinidae, at depths exceeding i,ooo 

 fathoms. The study of GuiviUea has shown how the degeneration of 

 the eyes takes place in the streptoneurous gastropods : the surface of 

 the retina becomes extremely reduced, the pigment has entirely dis- 

 appeared, and the epithelium of the eyeball has become uniform over 

 almost all its extent. 



2. The organisation of a group of Lamellibranchia, supposed to 

 be without gills, has been elucidated. It has been recognised that 

 their gills have been transformed into a muscular septum, whence the 

 name Septibranchia given to these organisms (Poromya, Silenia, 

 Cuspidaria=N etsra). Respiration is effected by the internal surface of 

 the mantle in the supra-septal chamber ; the water enters therein by 

 paired orifices pierced through the septum, and only opening from 

 without into the chamber ; the current of water is produced by the 

 contraction of the septum. 



3. The study of the Lamellibranchia has permitted the establish- 

 ment of a phylogenetic classification based on the structure of the 

 gills, and now more and more adopted. It has also put us in the 

 way of recognising the general hermaphroditism of the Anatinacea 

 [Lyonsiella, etc.), and the fact that, among the Mollusca, hermaphro- 

 ditism is secondary and super-imposed on the female sex, 



4. Among the Polyplacophora, the collections of the " Challenger '*■ 

 have given us a good acquaintance with forms that have but few 

 pairs of gills— eight or even six — notably Leptochiton benthns, where all 

 the gills are placed, with the smaller ones in front, at the sides of 

 the anus, behind the foot, in a sort of branchial chamber. This'fact,. 

 combined with others that we know, has led to the opinion that 

 among the Amphineura, which are the most archaic molluscs, there 

 originally existed numerous gills down the whole length of the body ;. 

 then that, in certain forms, their number has diminished from front 

 to back, only the posterior pairs persisting, and the last of all being 

 the only one that is preserved in other molluscs (Pleurotomariidae^ 

 Fissurellidae, Haliotidae, Lamellibranchs, and dibranchiate Cephalo- 

 pods) ; in Nanlilns the last two are preserved. 



5. Among the surface organisms, the very numerous " Pteropods" 

 collected have, by the facts of their organisation, enabled us to show 

 that the class Pteropoda must disappear, and that we must, with 

 Souleyet and Boas, place the animals so called in the Opisthobrancliia. 

 It has been possible to fix the precise position of the Gymnosomata, 

 which come from the Aplysioidea, and to confirm that of the 

 Thecosomata, which come from the Bulloidea. The special case of 

 torsion recognised in the straight Thecosomata is one of the grounds, 

 for believing that the Opisthobranchs — indeed, all the Euthyneura — 

 have undergone an untwisting, contrary to the twisting of the 

 Streptoneura or Prosobranchia, and that they are derived from 

 streptoneurous ancestors. 



