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 Plcurococcacece 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 Phurococcacece of which the 

 cells are not associated in definite colonies, but 

 separate on division, are represented in the sea by 

 the genus Zoocldwella. 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 Zoochlorclla may 

 live (in 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. 



