THALLOPHYTA— ALG.E 



37 



of the chroinatopliores is very constant in the several species. 

 In some of them cm*ious bodies called i^ijrenoids are found, 

 which are probably connected with the assimilative processes. 



The group exhibits considerable variety in its modes of sexual 

 reproduction. In the simplest forms that show sexuality, the 

 gametes are not distinguishable into male and female. In 

 Ulothrix {fig. 803) the contents of some cells break up into a 

 number of ciliated masses of protoplasm which escape from 

 the cell, and, after swimming about for a while, conjugate in 

 pairs. In the Z^^gnemeae and Mesocarpeae the gametes are 

 solitary and not motile, and do not escape from the cells in 



Fig. 787. 



Fig. 786 



Fig. 788. 



Fig. 786. Autlieridia, a, a, ou the branched hairs of the male couceptacle. 

 After Thuret. — -Fig. 787. Oogouium with the oospheres fully separated, 



and disengaging themselves from their coverings. After Thuret. 



Fig. 788. An oosphere without a celhdose coat being fertilised by anthe- 

 rozoids so as to form an oospore. 



which they are formed. In Cutleria the gametes are dissimilar 

 in size, but both are ciliated. The larger comes to rest soonest, 

 and one of the smaller fuses with it. More completely differen- 

 tiated gametes are found in higher forms {figs. 787 and 789) ; 

 oospheres are developed in oogonia and antherozoids in au- 

 tlieridia. "When the gametes are alike the reproduction is called 

 isoganious ; when they are different in size and behaviour it is 

 said to be oogamous. In the Rhodophyceae the aiitherozoid is 

 not ciliated and there is no differentiated oosphere. Instead of an 

 oogonium the female organ is known as a cai'jjogonium. It is 

 fre(xuently multicellular, and fertilisation is brought about 

 through a filiform or elongated cell known as a trichogyne. 



