THE COLLAR-CELLS OF HETHROCGLA. 11 
It appears to be annual, in common with S. ciliatum, 
Halichondria panicea (cf. Johnston, 1, p. 92), and Hy- 
meniacidon sanguineum. The rocks were covered from 
December to March of 1894 with large specimens of these four 
species; in September of the same year there were in some 
localities crusts of Halichondria, but it was for the most 
part difficult to find any sponges, except that careful search 
revealed a large number of very minute Sycon. Iam informed 
that a general absence of littoral sponges was noticed also in 
the autumn of 1893. Carter (No. 2) states that S. com- 
pressum breeds in May (larve at Plymouth July 18th, 
1895). 
This species is the best suited of all I know for examination 
under high powers during life. Its collar-cells are among 
the largest, if not as large as any known. Its strong radial 
spicules give a convenient consistency without impeding the 
razor ; they also protect the section from being crushed on the 
slide. Such sections are necessarily of great thickness as 
compared with paraffin sections, but the chambers of the sponge 
are so wide and extensive that rows of collar-cells can always 
be found standing out freely either against the light or against 
quite transparent tissues. On the rocks above mentioned 
S. compressum is habitually left for an hour or two at every 
ebb-tide to live on the water contained. in its canal-system ; 
the conditions of life under the cover-slip are therefore only 
partially unnatural. In experience, unless the slide, razor, or 
finger holding the sponge have been dirty, the flagellar motion 
will continue two to two and a half hours after covering, though 
changes of form, detailed below, become apparent after about 
a quarter of an hour. 
I have not yet used a gas-chamber, the sections having been 
merely placed in sea-water between an ordinary slide and 
cover-slip. Using a Leitz ;4, oil immersion with Zeiss oc. 3 
(old system) the collars and moving flagella appear with 
diagrammatic distinctness. I employed an Abbé condenser 
and blue glass, with incandescent gauze light focussed exactly 
on the object. About fifty living sections were examined, 
