West & West . — Observations on the Conjugatae. 31 
All Conjugates are surrounded by a definite mucilaginous 
envelope. In the great majority this covering is very thin, 
but in others it is profusely developed, e. g. Zygnema ano- 
mcthim , Hyalotheca mucosa , Staurastrum tumidum , 5. longi - 
spinum , &c. ; and even in those species in which it is normally 
almost absent it is occasionally developed to a large extent. 
We have seen very extensive mucilaginous envelopes round 
Closterium Lumda , Penium Libellula , and Cosmarium ovale , 
species which are normally destitute of such extensive invest- 
ments. No doubt this mucus serves in many cases as a means 
of attachment, as we have seen as many as a dozen specimens 
of Staurastrum tumidum attached to one leaf of Utricularia 
minor , and in the case of many of the species which occur on 
dripping rocks, this mucus is absolutely necessary for the 
purpose of attachment ; but an equally important use is 
probably that of protection from epiphytes and parasites 
(Chytridiaceae, &c.). With regard to the nature of this 
gelatinous investment, it must be considered either as a secre- 
tion or a mucous condition of the outer layers of the cell- 
membranes. Klebs regards it as quite independent of the 
substance of the cell-wall ; and as the cell-membrane of most 
Desmids is perforated by a large number of minute pores 
— excessively minute in some and not visible although 
perhaps present in others — one would be inclined to regard 
this mucous envelope as a secretion. But in some species 
of Zygnema it seems to us to be partly if not entirely due to 
the diffluent outer layers of the cell-wall, and this may be 
also true for some Desmids. In an almost pure gelatinous 
gathering of Cosmarium cymatopleurum , var. tyrolicum , many 
of the specimens were seen casting the outer coats of the cell- 
membrane, and in some cases many such successive coats 
could be seen round each individual gradually fading into 
the mass of jelly in which the plants were imbedded. This 
gathering was from the vertical face of a dripping rock. We 
have also noticed Cosmarium pyramidatum , when imbedded 
in a gelatinous mass of Desmids, casting its outer cell- 
membrane in a similar manner. 
