10 GEORGE BIDDER. 
PREFATORY REMARKS. 
The feeding experiments referred to in this paper were 
performed on Leucandra aspera and Sycon raphanus at 
the Naples Zoological Station,—some during an occupation of 
the Cambridge University table in 1887-8, some during later 
opportunities for work there, which I owe to the great kindness 
of Professor Dohrn. The observations on living cells were 
made chiefly on Sycon compressum at Plymouth; they 
were undertaken largely on the stimulus of the paper (19) by 
Vosmaer and Pekelharing. Some months during which Mr. 
Sedgwick has been good enough to allow me to work in his 
laboratory I have devoted to reviewing my permanent pre- 
parations of all species. Except where otherwise stated, the 
collar-cells of S. compressum are described below, this 
species having been preserved with the greatest care and 
success. 
Sycon raphanus grows abundantly on the walls of the 
tanks of the Naples Zoological Station. It differs here from 
the varieties ordinarily met in the possession of a very long 
fur of fine linear spicules. It has the obvious advantage that 
physiological experiments can be made in surroundings natural 
to it; on the other hand, it is rather small and soft for free- 
hand living sections, and its collar-cells are comparatively 
small. 
Leucandra aspera (var. gigantea, Vosm.) breeds in the 
port of Naples. It has the advantage of great size, large 
collar-cells, and a robust constitution habituated to the most 
poisonous surroundings; but its huge longitudinal spicules 
render free-hand sections practically impossible. It is very 
remarkable that in impure water it throws out a fur of fine 
spicules like that possessed by S. raphanus (var. aquariensis 
nova); it has occurred to me that this may be a filter against 
bacteria. 
S. compressum grows abundantly on the tidal rocks 
within ten minutes’ walk of the Plymouth Biological Station. 
