Thraustotheca , a Peculiar Water-Mould. 
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elastic restraining membrane, combined with the mutual attraction and 
superficial viscidity of the newly-formed spores, seems to explain quite 
adequately the sporangium dehiscence and spore liberation that characterizes 
Thraustotheca . Moreover, it is probable that the factors which affect spore 
liberation in this genus may play at least a contributory part in other 
cases. 
Mutual Attraction of the Sporangiospores. The escaping sporangio- 
spores of Thraustotheca show a mutual attraction resembling that which in 
a more pronounced form causes the aggregation of Achlya sporangiospores 
into a hollow sphere. Hartog (12 and 13) was the first to maintain that 
this phenomenon in Achlya was due, not to the embedding of the spores 
in a globule of gelatinous material, but to ‘adelphotaxy’, a form of 
irritability which he defined as £ the tendency of spontaneously motile cells 
to assume definite positions with regard to their fellows ’. As defined, this 
term is not applicable to the non-motile sporangiospores of Thraustotheca. 
Since, however, the ciliation and auto-motility of Achlya is not indisputably 
established, and since in A. paradox a, Coker, the escaping sporangiospores 
show only an irregular aggregation quite comparable to Thraustotheca, it 
seems justifiable to extend the term to include the phenomenon in this 
genus. 
The precise nature of adelphotaxy is at present unknown. It depends 
seemingly on the life of the spores; since Humphrey (15) found that the 
spores of Achlya fell apart when killed with osmic acid fumes at the precise 
moment of emergence, and the writer has found a like condition to obtain 
in Thraustotheca. This mutual attraction, moreover, is effective only when 
the spores are in contact or separated by a very minute space. In 
Thraustotheca, sporangiospores separated during escape by more than one 
half their diameter remain apart; and in Achlya Hartog (12) and Coker (8) 
have noted that an escaping sporangiospore, if separated from its fellows 
by its length, fails to take position in the hollow sphere, but moves off. 
Sharp distinction must be made, however, between adherence of the 
sporangiospores from mutual attraction and from the glutinous character 
of the newly-formed walls. In both Achlya and Thraustotheca the 
attraction is of brief duration ; but the adherence of the hardening walls 
of the appressed spores holds them firmly even after they have emitted 
zoospores, and are but empty cysts. 
Relationship. Although the exact relationship of Thraustotheca is 
largely a matter of conjecture, certain facts seem of significance in this 
connexion. A comparison with Dictyuchus gives scant basis for the 
customary taxonomic association of the two genera. The sporangia, both 
in structure and in method of spore liberation, are distinctly different ; 
while the sexual organs of the three authentic species of Dictyuchus are 
quite unlike those of Thraustotheca. To Achlya, however, Thraustotheca 
