GROUP I. THALLOPHYTA : ALGJ1. 227 



rived from distinct gametangia ; and in Dasycladus, only between 

 planogametes derived from gametangia borne by distinct indi- 

 viduals. Hence it appears that the planogametes formed in any 

 one gametangium of Acetabularia or of Ulothrix are all of the 

 same sex ; and, similarly, that the planogametes produced by any 

 one individual of Dasycladus are all of the same sex; but, in the 

 absence of any external differentiation, it is impossible to dis- 

 tinguish between the male and the female planogametes. 



The sexual cells are aplanogametes, planogametes, oosplieres, and 

 apermatozoids (see p. 80) ; though they differ widely in various 

 respects, they agree in being nucleated masses of protoplasm 

 destitute of a proper cell-wall. 



The aplanogametes are characterised by the absence of cilia 

 and of any defined form; they are confined to the Conjugates 

 and Diatomaceae. 



The planogametes are somewhat pear-shaped, the anterior more 

 pointed end being destitute of the chromatophores which are pre- 

 sent in the more rounded portion. They have two cilia which 

 are inserted, in the isogamous Chlorophyceae, at the pointed end 

 of the cell ; in the isogamous Phaeophyceae, laterally at the junction 

 of the anterior colourless portion with the posterior coloured por- 

 tion of the cell. In conjugation, the planogametes first come into 

 contact by their colourless anterior ends. Like the zoospores, the 

 planogametes have commonly a red pigment-spot (see p. 102). 



The oospheres are spherical cells, usually containing chroma- 

 tophores either throughout their whole substance, or leaving a 

 colourless area on one side, the receptive spot, at which the sper- 

 matozoid enters in the process of fertilisation (e.g. (Edogonium, 

 Vaucheria, Sphaeroplea ; see p. 116). 



The spermatozoids may be somewhat pear-shaped, resembling 

 the zoogonidia of the plant, but smaller (e.g. Coleochsete, QBdogo- 

 nium) ; or they may be more elongated and club-shaped (e.g. 

 Sphaeroplea, Volvox) ; or still more elongated and spirally twisted 

 (Characeae). They usually bear two cilia at the pointed end ; but 

 in Vaucheria, Volvox, and the Fucaceae, they are inserted laterally; 

 in (Edogonium there is a circlet of cilia round the pointed colour- 

 less end. They are faintly coloured, in the Chlorophyceae usually 

 yellow ; they sometimes have a red pigment-spot near the insertion 

 of the cilia (e.g. Volvox, Fucaceae). 



The male cells of the Floridece and of the Dictyotaceae are 

 peculiar on account of the absence of cilia, and are distinguished 



