160 GUY C. ROBSON 



some species of the genus and the general tendency in the group 

 to show a transition from a marine to a fresh-water hahit render 

 them a peculiarly interesting group and worth an intensive 

 study. In the last place the recent discovery of Partheno- 

 genesis in Paludestrina jenkinsi (Boycott, 4) makes 

 a closer study of the kindred species necessary. 



Our knowledge of the European Paludestrinidae includes 

 good accounts of the anatomy and histology of Bythinella 

 dunkeri (Bregenzer, 6) and Vitrella quenstedtii 

 (Seibold, 20), and more incomplete descriptions of part of the 

 anatomy of P . u 1 v a e (Henking, 10) and P . jenkinsi 

 (Eobson, 16). In spite of this amount of work a good deal 

 remains to be cleared up as to the structure of these animals. 



The material employed was obtained from tidal ditches at 

 Leigh-on-Sea, Essex. It was fixed in Bouin's solution after 

 the shell had been carefully cracked away so as to expose the 

 columella. Eeconstruction models of various organs were 

 made. A rapid method, which may be capable of improve- 

 ment, was devised, in which the usual plates were made up of 

 modelling-clay mixed with varying proportions of glycerine 

 and water and rolled out on pieces of thin paper. This mixture 

 can be graded to give a harder medium than Plasticine, and 

 is therefore more suited to making models of such parts as 

 contain delicate ducts, nerves, &c. The paper, if cut larger 

 than the plate, allows of rapid handling and can be cut away 

 after the plate is in position. The surfaces and edges of the 

 plates can be easily painted with water-colom-s. 



The author is indebted to the late Dr. W. G. Eidewood for 

 suggestions as to reconstruction models, and to Professor Paul 

 Pelseneer for information upon the general morphology of the 

 Taenioglossa. 



2. Structure. 



1. The Alimentary System. 



The oral cavity (Text-fig. 1) is usually deep and narrow. 

 Ventrally it exhibits a pair of lateral diverticula which are 

 sometimes forked. In general it agrees with that of other 



