On Platydorina. 425 



ing in the living condition, as a rule, no trace of differentia- 

 tion. The gelatinous nature of the substance is shown by the 

 great numbers of bacteria which swarm within it in morilnind 

 specimens. Colonies killed in formalin and stained in Dela- 

 field's haematoxylin exhibit a difference in the intensity of 

 coloration, indicating the presence of a denser peripheral 

 layer or sheath 3-4 /< in thickness (PI. XXXVIII., Fig. 1, 4, 

 }}. sh.). This is apparent along the edges of the colony, and 

 presumably extends over its faces. In several living colonies 

 a granular differentiation of this outer layer was noted about 

 the margin. This sheath is similar to that of Eudorina and 

 Pleodorina, but shows no trace of the concentric layers so 

 prominent in Pandorina. 



One of the most characteristic features of the colony is the 

 presence, upon the posterior border, of 3 or 5 projections or 

 tails, which are merely extensions of the sheath. Colonies of 

 16 cells have but three tails, while those of 32 cells have 

 uniformly five. These projections are bluntish finger-like 

 processes without structural differentiation, tapering some- 

 what to a rounded or pointed end. Occasionally the outer- 

 most pair, and more rarely the inner one, are slightly diverg- 

 ent. In the 16-cell colony there are two latero-posterior 

 tails and one median one (Fig. 2), the former being better 

 developed, and measuring 15 to 20 /< in length. The median 

 tail is variable in length, being sometimes a mere rudiment 

 appearing on a slight elevation on the margin. Its average 

 length is about one third that of the adjacent pair, though it 

 occasionally attains two thirds their length. The latero- 

 posterior tails are upon each side of the colony directly 

 behind the marginal row of cells, while the median tail is 

 midway between the central rows. In the 3 2 -cell colony 

 (Fig. 1) there is, in addition to the three tails above noted, 

 another pair wliich may be designated as the lateral pair. 

 These tails are slightly divergent, arising at the outer pos- 

 terior angles of the marginal row of cells, between the last 

 transverse quartet and the last sextet of cells. They are 

 from 10 to 15 /< in length, and are often of the same size as 

 the median tail of the colony. The other three tails occupy 



