41 



ON THE CONNEXION BETWEEN STYLE-SAC AND INTESTINE 

 IN GASTROPODA AND LAMELLIBRANCHIA. 



By Guy C. Robson, M.A., F.Z.S. 



liea] 13th January, 1922. 



(Published by permission of the Trustees of the British Museum.) 



So much has been written in the past upon the crystalline style in 

 the Mollusca that some excuse is necessary for a further excursion 

 into this subject. Some observations which I have recently published 

 seem, however, to be of sufficient interest to warrant fresh 

 morphological discussion on this structure. 



The problem for discussion is the significance of a narrow, 

 longitudinal slit placing the sac of the crystalline style iu com- 

 munication with the pyloric part of the intestine. This com- 

 munication between the sac and the intestine has been described 

 previously in the Lamellibranchia, and I have been able recently 

 to demonstrate its occurrence in the " Hydrobiid " genera of 

 Gastropoda, Paludestrina and Hypsobia (Robson, 1920-21-22). 



The relation between style sac and intestine in the Lamelli- 

 branchia has been very fully discussed by Matthias,^^ who 

 distinguishes three groups within that class. In these we find a 

 progressive separation of the pyloric and coecal (style sac) elements 

 of the stomach. In forms like Leda and Yoldia Area, Ostrea, and 

 the Septibranchia the pylorus and style sac are in wide and open 

 communication with each other. In Modiolaria and Jouannetia 

 this communication is very much restricted, only a narrow cleft 

 remaining. Finally, in Teredo, Pholas, Dreissensia, and others, the 

 sac and the pylorus are completely separated. E. Ghosh ® has 

 described several apparent instances of the second condition in the 

 Solenidse, and has discussed them with regard to the Lamellibranchia 

 as a whole. His account is a little puzzling in several cases. Under 

 Solen (e.g. p. 52) he says : " Coecum arising from the ventral aspect 

 of the pylorus," and does not specify whether there is an open com- 

 munication, though one might suppose from his introduction (p. 50) 

 that there is not. The same lack of precision is to be noted under 

 Subcultellus (p. 61). 



Burne ^ has described a condition representing the first of Matthias' 

 stages in Anatina elliptica. 



The occurrence of a style and style-sac in Gastropoda has 

 been discussed by several authors, notably by Moore ^^ (1898) and 

 M. F. Woodward ^i (1893). A summary of the known distribution 

 and modifications of these structures has not been given, so that I 

 take this opportunity of drawing together all the records that have 

 been accessible to me. The following list is probably not exhaustive, 

 though it covers a wide enough field to give an idea of the distribution . 



