88 Sturch . — Harvey ella mirabilis ( Schmitz & Reinke). 
and the division and growth of the peripheral cells goes on, 
the three or four cells immediately beneath the growing 
distal cell of the chain are generally small, gradually increasing 
in size ; thus there is a 3- or 4-celled peripheral layer, the 
cells of which pass over gradually into the larger cells of 
the central mass of the parasite. 
This growth continues until the external part of the Hav- 
veyella is in the form of a more or less hemispherical cushion- 
like body, varying in size up to about 2 mm. in diameter. 
This cushion is, when fresh, often perfectly white, but some- 
times it is of a brownish colour, owing to the external mem- 
brane and a few of the peripheral cells having become 
stained a dark brown. If a perfectly white specimen is kept 
in a bottle of sea-water for a day or two, it becomes stained 
in this way. When dried, all the specimens are of nearly 
the same colour as the dried Rhodomela- stems to which they 
are attached, very dark brown to black. This brown colour 
is, I believe, taken up by the external membrane and cells 
of the parasite, possibly from neighbouring dead cortical cells 
of the host. 
When mature, Harveyella mirabilis appears to the naked 
eye as a small hemispherical cushion on the Rhodomela- stems 
(Fig. 4). Very frequently three or four of these bodies are 
situated close together, often encircling the stem ; sometimes 
they are close together, but each one is distinct ; at other 
times they are more or less joined. They are generally the 
product of one original spore, the external portion being 
developed wherever the internal filaments reach the surface 
of the host. If very many internal filaments reach the surface 
close together, they produce one large external thallus or 
frond : but if separated by some distance, they give rise to 
several smaller fronds, which may more or less coalesce as 
they increase in size. Thus the thallus, of which a section 
is shown in Fig. 3, was probably formed by the coalescence 
of at least three smaller fronds, all of which were originally 
derived from the same spore. Thus, when mature, Harveyella 
mirabilis consists of : — 
