OSCULA AND ANATOMY OF LEUCOSOLENIA CLATHRUS. 481 
directly by the flattened ectoderm-cells. In some sections 
(6 5) it looks as if there was an intermediate form of cell; 
but I am convinced that this is only due to ordinary collared 
cells being cut obliquely, so that only their bases are seen. A 
short distance above where the collar-cells stop, the sphincter 
is seen projecting like a ledge into the interior. Above it the 
ectoderm goes on for a considerable distance. The height of 
this oscular margin, formed only of two layers of ectoderm 
with some spicules and amoeboid cells between, is really 
remarkable. 
Fig. 7 @ represents the sphincter of one of the larger oscula 
in section (the two sides of the oscula are of course not drawn 
at their natural distance apart, for then they would have to be 
separated by more than the length of the whole plate). 7 dis 
another section, rather thick, from the same series, showing 
the sphincter, which was a little crumpled, cut tangentially 
and obliquely. This osculum had been hardened with a satu- 
rated solution of corrosive sublimate in absolute alcohol, and 
the sections stained on the slide with borax carmine first, and 
then with hematoxylin, a method which I find exceedingly good 
for showing the ectoderm. Fig. 8 represents a transverse sec- 
tion of the sphincter of another osculum, prepared by the 
osmic-picro carmine-hzematoxylin method. 
The sphincter, as can be seen, projects as a ring-like ridge 
into the interior ofthe osculnm. We shall consider its minute 
structure presently. When this sphincter contracts it closes 
the osculum, which then, in the large colonies, has a very 
characteristic shape, which I do not know how to describe 
better than by comparing it to the human breast (fig. 3). In 
such a breast-shaped osculum the nipple is formed by the ecto- 
dermal margin of the osculum, and at the base of the nipple 
one finds the contracted sphincter. Fig. 9, a, 6, and c, repre- 
sent sections from a series through the osculum represented 
in fig. 3, hardened in abs. subl.,1 and stained with borax 
carmine and hematoxylin. 
1 T use this as a convenient abbreviation for a saturated solution of corro- 
sive sublimate in absolute alcohol, 
