OSCULA AND ANATOMY OF LEUCOSOLENIA CLATHRUS. 491 
exceedingly distinct. I find the very greatest difference in this 
respect between Leucosolenia clathrus and L. coriacea 
occurring at Plymouth. 
To sum up the results obtained : 
Leucosolenia clathrus is not permanently lipostomous, 
but has very large and distinct oscula. 
These oscula are provided with a sphincter by which they 
can be completely closed for a time, apparently as a protection 
against unfavorable external conditions. 
Haeckel’s four varieties of the sponge are only different 
states of contraction, and are no more zoological varieties 
than a polyp with contracted tentacles is a variety of a polyp 
with expanded tentacles. 
The many-layered endoderm is also only a temporary con- 
dition, the mechanical result of the contraction of the whole 
sponge. 
The contractile elements in all cases are the flattened ecto- 
dermal epithelium. 
In conclusion, it is my pleasant duty to express my thanks 
to the staff of the Naples Zoological Station, and especially to 
my kind friend Sig. Cav. Lo Bianco, without whose help this 
work could never have been done. 
Naptes ; November 10th, 1891. 
ADDENDUM. 
Wuite the above was in the press, a work by von Lendenfeld 
has appeared, entitled “Die Spongien der Adria.—I. Die 
Kalkschwimme” (‘ Zeitschr. f. wiss. Zool.,’ Bd. liii, Heft 2, 
pp. 185—821, Taf. vili—xv; and Heft. 3, pp. 361—433), 
containing a detailed account of Ascetta clathrus (pp. 
paper, but as my figures might be thought to be erroneous I will only say 
that in freshly preserved material of the sponge the ‘‘ Metschnikoff’s cells” 
only occur sparingly, the predominant form of the ectoderm being flattened 
epithelium ; and I have almost conclusive evidence to show that the “ flask. 
shaped cells ” are only the contracted condition of the flat cells. 
