THALLOPHYTA. 651 



leave an opening. A certain amount of the protoplasm of the oosphere is extruded 

 at this spot, and forms a sort of canal of mucilage, through which a spermatozoid 

 passes in fertilization. The spermatozoids are produced one or two in an anther- 

 idium, which are short cells poor in chlorophyll, formed by the repeated division of 

 certain cells of a thread. The spermatozoids resemble small zoospores. In some 

 cases they are not produced directly from the cells of a thread, but the latter give 

 rise to special zoospores called androspores, which escape and settle either on or in 

 the immediate neighbourhood of an oogonium. The androspore then germinates, 

 producing a small structure called a dwarf male. This consists of a very few cells, 

 one or more of which become antheridia, and opening by a lid, gives rise to sper- 

 matozoids, one of which pierces the canal of the oogonium, and fuses with the 

 oosphere. The oospore, which is of a red or brown colour, produces four zoospores 

 in germination. 



Bulbochcete is a genus resembling (Edogonium in its life -history, but consists 

 of a branched thread, only the basal cell being capable of dividing. The cells 

 bear characteristic hairs, swollen at the base, whence the name of the genus. 



Species of (Edogonium and Bulbochcete, which are both genera of considerable 

 size, are found in our ponds and ditches. 



The next three families are distinguished from all other Confervoidece by pos- 

 sessing more than one nucleus in each cell. 



Cladophoraceoe. Cladophora is a very widely distributed genus, inhabiting both 

 fresh and salt water. A great number of species have been described, but it is 

 doubtful whether many of the forms are entitled to specific rank. 



The thallus has a very characteristic habit. It is fixed below by an elongated 

 basal cell, and is profusely branched, sometimes forming a spherical mass. The 

 elongated cells possess parietal chromatophores, which cover the whole cell-wall, 

 and possess many pyrenoids. Sometimes the chlorophyll-layer is separable into 

 distinct angular plates. There are many nuclei in the layer of protoplasm imme- 

 diately within the chlorophyll. 



Many zoospores are produced in each cell. The nuclei divide a good deal, the 

 pyrenoids disappear, and the protoplasm then divides into a number of separate 

 masses, each of which forms a single zoospore with either four or two flagella. 

 Gametes with two flagella are formed in many species quite like the zoospores. 

 The zygote germinates directly to form a new Cladophora-pla^i. 



The Cladophoraceae show on the one hand a transition to the Siphoneae, and on 

 the other, through certain genera with unbranched thallus and few nuclei in each 

 cell, to the Ulotrichacese. 



Gomontiacece. Gomontia polyrhiza is an isolated form which perforates the 

 shells of various marine molluscs, such as the whelk, the oyster, the mussel, &c. 

 The thallus radiates on the surface of the shell, and sends branches into the sub- 

 stance, gradually disintegrating it. Certain branches become zoosporangia or 

 aplanosporangia; these lose their attachment to the thallus and form fresh rhizoids. 

 The zoospores are pear-shaped, and germinate directly to form a new thallus. 



