2lS 



ON SOME SPECIES OF PISIDIUM IN THE SWEDISH 

 STATE MUSEUM. 



By nils HJ. ODHNER. 



(Read before the Society. May 4th, 1921). 



The discrimination of the species belonging to the genus Pisidiuvi 

 has always been fraught with great difficulty. To some extent this 

 fact has been due to the smallness, which rendered a careful examin- 

 ation of their entire organization impracticable for the purpose of 

 determination. To base the specific characters exclusively on the 

 shell — a general practice in the past — has led to much confusion, as 

 the shell is subject to considerable variation, and presents no abso- 

 lutely constant characters. 



In 1908 I found ^ that the soft parts offer important evidence with 

 regard to the taxonomy of the group ; thus the gills show differences 

 in their formation, inasmuch as the posterior one may be present 

 though more or less reduced, or it may be totally absent. Subsequent 

 researches have confirmed this opinion. I intend soon to publish my 

 investigations : on this occasion I will only call attention to some 

 facts of special interest. 



Even the anatomical characters, however, are not constant. For 

 example : the posterior gill, even in species where it is normally 

 present, may be reduced or totally absent. These conditions of tlie 

 gills occur in connection with reduced body size, and are probably 

 due to unfavourable environment. That such a lack of the posterior 

 gill is not restricted to the juvenile stage, where the anterior gill 

 originates earlier than the posterior one, is proved by the fact that 

 dwarfs often have fry within their gills. A tendency to a sort of 

 neoteny thus seems to be prevalent in the genus, at least in certain 

 species, e.g., P. siibtruncatum and P. milium, where I have observed 

 a partial or complete reduction of the posterior gill in small forms 

 with fry. 



The reproduction of the Pisidia is of great interest, since self- 

 fertilization seems to be common — perhaps the rule in certain species. 

 Attempts to cultivate them have yielded the following results. In 

 the autumn of 1919 a number of Pisidia were caught and kept living 

 throughout the winter in a small aquarium. During this time the 

 animals reduced their functions of life to a minimum ; they continued 

 wholly withdrawn into their shells, and nothing but the pulsations of 

 the heart indicated their living state. Among them was a single 

 P. miliian and a single SpJmriiim corneum. A great many individuals 

 perished during the winter, and few survived until the spring. Then, 



I Die Mollusken der Lappli4iidischen Hochgebirge. Naturwiss. Unters. Sarekgebirges 

 ill Scliwedisch-Lappland, gel. v. Dr. A.xel Haiiiberg, IV, 2, Stockholm, 190S. 



