144 Barber. — On the Structure of P achy theca. 
of the ‘ stoppers ’ appears to indicate a comparatively highly 
differentiated organism. One of the questions of greatest 
interest is, whether we have here to deal with a single plant 
or a case of symbiosis or parasitism. If all the cell-filaments 
and the oval bodies belong to one organism, the plant is, 
I believe, without parallel among existing Algae. If, on the 
other hand, the central filaments prove to be parasitic on, or 
symbiotic with, the rest of the parts, we are not altogether 
without similar cases. Mr. Thistelton Dyer has referred me 
to a species of Rivularia which is penetrated in a similar way 
by a Cladophora. This species, which occurs in Belvedere 
Lake, Mullingar, further resembles Pachytheca in its possessing 
a stony consistence : it forms small round pebbles on the 
bottom of the lake. 
It is, however, easy to construct any number of hypotheses 
regarding so puzzling an organism. The oval bodies may 
be regarded as being the skeleton of a siphonioid form, and 
the possible resemblance to Cymopolia has been suggested. 
Until further sections are obtained it does not appear prob- 
able that there will be any easy solution of the matter. 
In conclusion, a curious resemblance may be pointed out 
between the section of Pachytheca and a transverse section of 
the stalk of Fur cellar ia fastigiata . In the latter section there 
are three zones as in Pachytheca : a central portion containing 
a colourless matrix penetrated in all directions by cell- 
filaments ; a cortical zone of radiating cell-rows ; and a zone 
of large spherical cells separating the two. The cortical cells 
are continuous with the zofie of spherical cells, but the cells 
of the central portion appear at first not to join the latter. 
On the contrary, they evidently push themselves in between 
the spherical cells. There are, as in Pachytheca , numerous 
bars stretching between two neighbouring spherical cells, and 
the penetrating central filaments appear to wind in between 
these bars. There appears at first to be no connection 
between the central filaments and the spherical bodies ; a 
longitudinal section at once shows, however, that each 
spherical body is connected with a cell of the central portion 
