632 SCIENCE PROGRESS 



end of the cell is generally somewhat truncated, while towards 

 the back it narrows down slightly. The place of the cell-wall is 

 taken by a denser peripheral layer of protoplasm, which, how- 

 ever, allows of a limited change of shape (metaboly). A nucleus 

 occupies the centre of the cell, while the bell- or goblet-shaped 

 chloroplast, which contains a single pyrenoid, is situated at the 

 back end, and bears a red eye-spot at its margin. One or two 

 contractile vacuoles lie near the bases of the cilia. The only 

 method of reproduction observed is by means of longitudinal 

 division of the contents after the cilia have been drawn in. The 

 organism also possesses the power of encystment, a single 

 individual being formed from the cyst on germination. The 

 genus Pyramimonas (Dill 21, Griffiths 31) is a close ally and 

 agrees in many respects with Polyblepharis, but the cell is 

 characteristically pyramidal and has four wing-like protrusions : 

 there are, further, only four cilia, and the eye-spot (s) is situated 

 posteriorly (fig. 1, m). A sexual process has not been observed, 

 and reproduction appears to be effected solely by longitudinal 

 division, the process of constriction commencing at the back end 

 of the cell and generally taking place during the motile stage. 

 A genus has been recently described under the name of 

 Dnnaliella (Teodoresco 65a) which appears to be closely allied 

 to the two forms just considered. Like them it lacks a cell-wall, 

 has a bell-shaped chloroplast, longitudinal division, etc., but its 

 particular interest to us lies in the fact that a sexual process 

 between isogamous gametes has been established in this case. 



The most essential peculiarities of these three organisms, as 

 contrasted with Chlamydomonas and its allies, lie in the absence 

 of a cell-wall and the consequent power of change of shape, 

 in the prevalence of longitudinal division and in the power of 

 forming cysts. These are all features which are characteristic 

 of the Flagellata, as we shall see. On the other hand, in their 

 motile unicellular character, in the characteristic chloroplast, in 

 the formation of starch during assimilation and in the general 

 distribution of the cell-contents, Polyblepharis \ Pyramimonas and 

 Dimaliella show a very marked resemblance to Chlamydomonas 

 and the allied genera. This resemblance is heightened when 

 we recollect that certain species of Chlamydomonas show 

 longitudinal division and have the faculty of encystment (cf. 

 pp. 628 and 630). The genera Polyblepharis, Pyramimonas and 

 Dimaliella, which with one or two other, not very well known, 



