QUICK : NOTES OX ANATOMY OI' I'AI.nDKSl'RINA STAONAMS. 07 



'I'lie proportion of nialt-s to females cannot he detennined, as a 

 considerable number are infected by the cerceria; \i\vvcc of a parasitic 

 trematode. This was first pointed out to me l)y Mr. G. C. Robson 

 for a group of P. sfai^/ialis from Oxwich, viz., forty-eight out of fifty ; 

 and I have since found tlie condition, though in a much smaller pro- 

 portion of cases, in Swansea Ray specimens. The parasites occupy 

 the site of the gonad, and the remainder of the genitalia usually 

 cannot be traced, though one of the specimens had a well developed 

 penis. The larger and more tumid individuals that are definitely 

 sexed, however, are females, and the smaller more compressed shells 

 contain males. 



In both sexes a pallial tentacle arises just within the right mantle 

 margin, and is protruded during locomotion. In both sexes the left 

 cephalic tentacle is thicker than the right, and presents a crenulated 

 outline. The cilia are concentrated in the constrictions. The right 

 tentacle is smooth and uniformly ciliated. Both have a pigmented 

 ring, a short distance below the apex. 



As to salinity of water, the Swansea Bay F. stagnalis live in 

 practically pure sea water. At Oxwich, during neap tides, the fresh 

 water oozes into the pools, and a sample on analysis gave i'59% NaCI, 

 i.e. about 60% sea water. During rain and neap tides the proportion 

 of NaCl must be much less. 



F. Je?ik!risi \\we% at Oxwich in 0*035% NaCl, and at Llanrhidian in 

 o'i9% NaCl, but these sites are inundated at the highest spring tides 

 for a short time. A group of P. jenkinsi placed in 33% sea water on 

 July 6th, 1919, is still active on Se{)t. 13th. 



Compared with P. jenkinsi, the main points that stand out are the 

 occurrence of males and females, the laying of eggs, and the earlier 

 stage of development at birth, viz , veliger larvre ; the tentacular 

 asynuiietiy, the presence of a pallial tentacle, the frequency of infec- 

 tion by a trematode (^P. Je/ikifisi hom within a hundred )'ards of the 

 /'. si<rg//(///s are free from it), and the tendency of P. s/agnalis to the 

 sea side of brackish water, and of P. jenkinsi to the fresli water side. 



Parthenogenesis in Paludestrina jenkinsi from Brackish Water. — 



Koll()\vin<:!; up Prof. Boycotl's note on Parllienogenesis in freshwater /'. jenkinsi, 

 four individuals, I mm. long, from Llanrhidian Marsh, 0'I9% NaCl, were isolated 

 with all precautions on June 28th, 1919, and the vessels frequently searched with a 

 lens for accidentally introduced new born young, with negative results. On Sept. 5th 

 vessel A had one infant, Sept. 8th three infants, Sept. 12th five infants. \'essel C, 

 Sept. 7th two infants, Sept. 12th three infants. The mother C was then examined, 

 and on dissection contained thirty embryos in all stages of development. — II. E. 

 Quick, [Read brfote the Sodr'y, October iSth, 1919). 



