180 SEAWEEDS 
the sea. The cells of the thallus are motionless, 
and may occur singly or in colonies. There are no 
gametes or zoospores, and reproduction is exclusively 
a matter of cell-division in the ordinary vegetative 
manner. A number of Plewrococcacew are generally 
regarded as reduced forms allied to other orders. 
The Thallus of Prasiola is a membrane consisting 
of a single layer of which the cells are rectangular, 
while in Palmophyllum a rounded, lobed, and zoned 
frond is formed of roundish cells embedded in a 
gelatinous mass. The Plewrococcacee of which the 
cells are not associated in definite colonies, but 
separate on division, are represented in the sea by 
the genus Zoochlorella. This remarkable genus 
inhabits the bodies of various invertebrate animals, 
such as Radiolaria, &c., in which they have long 
been known as the “ yellow cells.” It has been con- 
tended by Brandt, Geddes, and others that there is 
here a true symbiosis or mutual partnership between 
the plants and their hosts, and that the animals are 
nourished on the products of assimilation by the 
plants, as in the case of the lichens, where the fungal 
portion subsists on the algal gonidia. It appears, 
however, from recent research, that while this is no 
doubt true enough, it is not the whole matter, and 
that the symbiotic relation is more partial or even 
occasional than was at first supposed. It has been 
demonstrated by Beyerinck that Zoochlorella may 
live Gn his experiments for several weeks) outside 
the bodies of animals, and Famintzin has observed 
their multiplication in a nutritive solution. An ex- 
tensive literature has arisen on this subject generally. 
