1921] NATURAL SCIENCES OF PHILADELPHIA. 117 



ever, it is extremely transparent, and there is some difficulty in 

 distinguishing it; a moderate enlargement, indeed, which allows of 

 a very small diaphragm, is better for its detection than very strong 

 magnificalion. 



During the Heliozoan state of rest, the flagellum remains quiet, 

 and nearly always with a peculiar shape, somewhat Hke a figure 8 

 (Plate V, fig. 7) ; from time to time, however, it is seen slowly to 

 unroll, and to perform one or two slight movements, soon coming 

 at rest again. 



This Heliozoan tyi^e is the less frequent form, and the animal- 

 cule never keeps it for a very long time. After a few minutes of 

 observation, the swinging of the flagellum grows quicker and more 

 frequent, and suddenly the whole body seems to be subjected to a 

 disturbance, a rupture of its internal equilibrium. It collapses, in 

 fact, and one might take it to be disorganized and dead; but after 

 hardly one or two seconds have passed, the whole mass, as by en- 

 chantment, lengthens into a fusiform body, having anteriorly a 

 quickly-moving flagellmn. 



The Httle flagellated being (Plate V, fig. 8), which from 10^ in 

 the Heliozoan state has now increased to ISjjl in length, and even 

 sometimes more, has the appearance of a fish, broader at the an- 

 terior part, and drawn posteriorly into an often very thin and long 

 extremity, which seems to result from the fact that the axial threads 

 have joined together and trail behind. The nucleus, very distinct, 

 has come forward, very near the anterior extremity, and is seen 

 surrounded by very small, hyaline granulations. The flagellum 

 comes straight from a slight excavation in the middle of the anterior, 

 rounded extremity. Several small contractile vesicles are visible, 

 one of them larger, near the posterior end of the body. 



The animalcule goes straight forward in a rapid course and swims 

 for a very long time, for hours probably; then suddenly it stops, 

 the flagellum becomes quiet and takes the characteristic figure 8 

 shape; again a commotion, which shakes the whole body, and rays 

 are seen to protrude from every part of the surface. In two sec- 

 onds, the Flagellate has turned into a Heliozoon. 



The transformation is effected here in the same way as we saw 

 it in Dimorpha tetramastix, but is much less evident to the eye. 



Bicoeca exilis sp. n. Plate V, figs, 9-13. 



This elegant little Flagellate was found, in February, 1918, rather 

 abundant in one of the aquaria of the Zoological Laboratory of 



