PRO TOCOCCOIDE.E 



411 



In the following paragraphs only the more remarkable or better 

 known genera are described. 



In Sciadiuni A. Br., made by some writers the type of a distinct 

 family Sciadiace/E, the peculiar mode of germination of the zoospores 

 gives rise to a remarkably complicated structure. Each individual consists 

 at first of a single elongated cylindrical cell. The green protoplasmic 

 contents of this cell break up ultimately into a number of biciliated 

 zoospores, which are set free by the upper })ortion of the cell -wall be- 

 coming detached in the form of a cap. 

 The zoospores do not, however, escape, 

 but 2:erminate while still attached to 

 the mother-cell, giving rise to a cluster 

 of smaller cylindrical cells springing 

 from the apex of the mother-cell. This 

 may go on until the colony consists of 

 as many as four generations, giving 

 the appearance of a minute branching 

 shrub. The zoospores of each genera- 

 tion are smaller than those of the pre- 

 ceding one, and it is probable that 

 those of the last generation, which escape 

 altogether from the parent-cell, are 

 conjugating zoogametes. It is pos- 

 sible that a form aUied to Sciadium may have been the starting-point of 

 the Siphonocladaceae, with which family it shows a certain affinity, as, for 

 example, with Valonia. 



Chlorothecium Bzi. (Malpighia, 1888, p. 250) occurs in the form of 

 palmelloid colonies with a thick and firm cell-wall on aquatic plants. 

 From the cells of these colonies are developed zoosporanges, or rather 

 gametange:^ without any alteration of their primitive form ; from each 

 gametange there escape from two to four swarm-spores, or occasionally 

 only one, each provided with a single cilium and a conspicuous red 

 pigment-spot. These swarm-spores are zoogametes, conjugating by 

 gradual fusion. After hibernating the contents of the zygosperm break 

 up into two masses, each of which escapes as a non-sexual zoospore^ so 

 that the zygosperm is itself a zoosporange. From these zoospores are 

 again formed the palmelloid colonies, in which form Chlorothecium 

 may multiply itself non-sexually without producing zoogametes. 



The position of HalospJuBra (Schmitz, ]\Iittheil. Zool. Stat. Neapel, 

 187S, p. 61) is very doubtful. Each individual is a minute green globe, 

 just visible to the naked eye, as much as 0*5 mm. in diameter, floating 

 on the surface of the sea, and bearing an external resemblance to Volvox. 



Fig. 



536. — SciadiHiii arbnscula A. Br. 

 (magnified). 



