PH AOPHYCEA 93 
The Geographical Distribution is throughout all 
seas, but the majority of the forms occur in the 
North Atlantic. Chordaria, Mesoglea, Castagnea, 
Myriocladia, Petrospongium, Leathesia, Myriactis, and 
Myrionema occur in British seas. 
ELACHISTACEA 
General Characters—In its most simple form the 
thallus consists of a mere tuft of branching filaments 
with basipetal growth in length, but cushion-like 
forms of thallus occur, composed of a basal layer 
clothed with densely packed upright assimilative 
filaments. The unilocular sporangia, mostly ovate 
or pyriform in shape, arise as lateral out-growths from 
the assimilative filaments, while the plurilocular spor- 
angia (giving rise to gametes, which in one case at 
least have been observed to conjugate) are produced 
either in place of such filaments or by the differenti- 
ation of a cell of one of them, or as an out-growth of 
such a cell. Considerable diversity occurs within 
the order in the development of the last-mentioned 
bodies. 
The Thallus in no case attains a greater stature 
than minute, filamentous tufts or patches, and in 
most cases is epiphytic on larger Algse, especially 
Fucacee, inhabiting in some cases even the con- 
ceptacles of these seaweeds. The rhizoids penetrate 
the tissues of the host-plants, and some of the forms 
send out runners, which traverse the tissues or creep 
over the surface, giving rise to new plants by this 
