THALLOPHYTES. 
resemblance in their mode of origin and subsequent development to those of the 
Pandorinege. When the coenobia multiply, a large number of zoogonidia are formed 
in each mother-cell, within which they move about for some time ; when they come 
to rest they congregate in some definite arrangement, and then continue their develop- 
ment unitedly, eventually again multiplying in the same way, as for instance in Pediastrum 
(Fig. 169). In Hydrodictyon utriculatum, which occurs occasionally in ditches, the mature 
zoogonidia^, these are in active 'creeping' motion; C the same family of cells 
4^ hours after its birth, 4 liours after tlie zoogonidia have come to rest; these have 
arranged themselves into a plate, vi'hich is already beginning to develope into one 
similar to A. 
^lant or ccenobium consists of a sac-like net several centimetres long, which is composed 
of a great number of cylindrical cells united at their ends so as to form a four- or six- 
cornered mesh. The ordinary mode of reproduction consists in the green contents 
of one of the cells of the net breaking up into from 7,000 to 20,000 zoogonidia which 
move about with a trembling motion within the wall of the mother-cell, come to rest in 
the course of half-an-hour, and then arrange themselves in such a way that, by their 
elongation, they again form a net of the original kind which is set free by the absorption 
of the wall of the mother-cell, and attains, in the course of three or four weeks, the size 
of the mother-plant. In other cells of the mature net the green contents break up 
into from 30,000 to 100,000 microzoQgonidia which at once leave the mother-cell and 
swarm about for some hours. The hypothesis has not yet however been confirmed 
that conjugation then takes place between these zoogonidia^; but when they come to 
rest they are spherical, invested with a firm cell-wall, and may retain their vitality for 
months when dried up if protected from light. After remaining several months at rest 
these resting-spores begin to grow slowly, and after they have attained a considerable 
size their contents break up into two or four large zoospores which come to rest 
after a few minutes, and assume a peculiar angular form when they have reached a 
considerable size, putting out horn-like appendages. In each of these so-called poly- 
hedra the green parietal protoplasm again breaks up into zoogonidia which move about 
for 20 or 40 minutes within a sac which protrudes out of the polyhedron. When come 
to rest they arrange themselves into a sac-like net consisting of from 200 to 300 
cells, but in other respects resembling one of the ordinary ones. In some of the 
polyhedra smaller and more numerous zoogonidia are formed, but these also unite 
into a net. 
[See note s7ipra, p. 251.] 
