A GROUP OF TURBELLAEIA. 51 



animal in the way of providing food, his reasons being that 

 closely allied species, living under similar conditions, do not con- 

 tain algae, and that solid food is ingested after the algal cells are 

 fully developed. The writer hopes to investigate this question 

 more fully, for Sekera's argument does not seem to be quite 

 conclusive. 



Sir J. G. Dalyell (1) wrote an account of this interesting 

 species in 1814, and states that it sometimes occurs in large 

 numbers, and then suddenly disappears. He found his specimens 

 chiefly in the spring, but some were found in the autumn. 



Mesostoma Spp. (PL 4, fig. 4). 



Some of the species of Mesostoma produce two kinds of eggs 

 thin-shelled and thick-shelled. The thick-shelled eggs, which 

 contain a large quantity of yolk, are produced in the late summer 

 and lie dormant during the winter. The young hatched from 

 these so-called " winter " eggs, when less than half the size of the 

 parent commence to produce thin-shelled eggs with but little 

 yolk. It is probable that these eggs are unfertilised ; they are 

 produced in great numbers and begin to hatch in April and May. 

 The young hatched from these eggs attain full development 

 and produce thick-shelled " winter " eggs, which have been 

 fertilised (14). 



There is some difference of opinion amongst observers as to the 

 precise nature of the life-cycle in this genus. See von Graff (17). 

 They vary in size from 3 to 15 mm. in length according to 

 the species and condition. They live in clear, still or slowly 

 flowing water and swim or creep over water-plants. Their food 

 consists of entomostraca, small worms, etc., which are sometimes 

 caught by means of slime threads. 



Bothromesostoma personatum (Schm.). 



Specimens of this species attain a length of about 7 mm. and 

 are easily identified by two white patches which look like large 

 eyes on each side of the "head." The rest of the body is either 

 grey or black. The writer has taken specimens on the leaves of 

 water-lilies and creeping on the surface film, at Staines and at 

 the East London Waterworks. The genus Bothromesostoma is 

 closely allied to Mesostoma, and like the latter produces both 

 summer and winter eggs. 



