OUHNER : ON PISiniUiM IN THE SWEDISH STATE MUSEUM. 219 



before the normal functions of life had begun, I isolated some in 

 separate glasses. The animals gradually became more active ; first 

 the intestine, which had been empty before, was observed to be filled 

 with green algfe ; and the liver, which earlier was greyish like the 

 entire animal, acquired the normal brown tint. The genital glands 

 soon afterwards began to appear as blackish masses in front of the 

 pericardium ; nothing of them had been visible before. 



In many of the specimens thus isolated, belonging to nitidii/ii, 

 casertannm, and milinin, as well as SphcBriuni corneum (of the two 

 last it will be remembered there were but single specimens under 

 cultivation), embryos were developed within the gills and later on 

 released. I'hough I had not opportunity to continue the cultivation, 

 I think I am justified in concluding that the young in these cases 

 were produced by means of self-impregnation. 



P. henslowaiiHiii and P. lilljeborgi were also isolated in the same 

 manner, but did not show any development of the genital organs, 

 probably because the stock to which they belonged had bred in the 

 autumn just after capture. 



It may be questioned whether autogamy is a common phenomenon 

 — perhaps the rule, at least in certain species — in the Pisidia ; yet 

 their ubiquitous occurrence may perhaps be a consequence thereof. 

 If this is the case, they should reproduce themselves along so called 

 "pure lines." For this-supposition speaks the fact that, as a rule, in 

 every fry-bearing specimen of Pisidiuin a complete conformity exists 

 between all the young, and these may be rather numerous ; I have 

 observed up to twelve or sixteen {P. obtusale) or even twenty {Jiiber- 

 nicuni) in one individual. Only in one species have I found a 

 remarkable exception to this rule ; in the case of P. henslowanum 

 the umbonal appendiculse vary considerably so that they may be well 

 developed or totally absent in different young within the same 

 mother ; this variation seems to suggest that this is a mendelian 

 character. Experimental investigations on such hereditary problems, 

 to which the Pisidia seem to be well adapted, would certainly give 

 interesting results. 



I take here the opportunity of drawing attention to a character of 

 the shell oi Pisidiuin, which has been misinterpreted and has given 

 rise to incorrect statements in literature. I refer to the fact that the 

 shells of the young are quite smooth, not hairy, and the same is the 

 case with all forms in adult stage. Many authors maintain, like 

 ^^'oodward, that the shell of Pisidium is " sometimes, especially in 

 young individuals and certain species, sparsely covered with short 

 hairs." But the "hairs "are something quite different, namely pores 

 leading into short canals, which penetrate, mostly perpendicularly. 



