A SIEVE-LIKE MEMBRANE IN LEUCOSOLENIA. 263 
distinct. These cells are best studied either by maceration of 
carefully fixed portions of the sponge, or by tearing up and 
examining living pieces. My macerations were done by fixing 
fresh pieces of the sponge in 3 per cent. osmic acid, and then 
soaking them in dilute glycerine, with or without previous 
treatment with } per cent. acetic acid or picro-carmine. After 
a time the cells may easily be separated by taps on the cover- 
glass with a needle. 
The shape of the cell in fresh specimens well hardened in 
osmic is elongated, as described above, but when teased up 
alive the cells become rounded (fig. 16), as is also the case in 
specimens hardened in Flemming’s fluid (fig. 22). In the 
living condition they may be observed to vary in shape, 
becoming, in fact, amceboid, as has been often observed. In 
living cells thus treated I have never been able to observe a 
trace of the collar. It appears to become completely retracted. 
Here I am in partial agreement with Topsent, who says of the 
collar-cells of Cliona, ‘‘Collerettes et cils sont retractiles 
comme les pseudopods de cellules améboides.”! I have always 
found the flagella quite distinct, but never the collar, though 
I have no doubt the former are also retractile. This retraction 
of the collar may be compared to that known to occur in 
Choanoflagellata.2, In macerations the collar is sometimes 
very low (fig. 17), but more often of considerable height 
(fig. 18). It is usually somewhat crumpled, and appears as two 
fine lines on each side of the flagellum. It is very seldom that 
the rim of the collar can be distinctly seen. I never found the 
collars of neighbouring cells joined together to form a “‘Sollas’s 
membrane;” I do not say this, however, as a disbeliever in the 
existence of this structure in some other sponges. I have 
sections of Halichondria panicea in which I can see it 
' «Contributions a l’étude des Clionides,” ‘Arch. de Zool. expér. et 
gén.,’ tome v bis (1887—1890), mém. iv, p. 27. 
2 Biitschli, ‘‘ Protozoa” in Bronn’s ‘ Thierreich,’ Bd. ii, Mastigophora, p. 
881: “ Wie bekaunt, ist der Kragen, wenigstens bei den Craspedomonadinen, ein 
gestaltsveranderliches Organ, ja er kann unter Umstanden ganz eingezogen, 
und wiederum neugebildet werden.” 
