334 WILSON. [Vou. IX. 
ectodermal area (ec. wn-p. ~.) which covers the spicular pole in 
the swimming larva. These cells of the swimming larva, as 
has been said, stain very intensely, and have too characteristic 
an appearance for their identity to be doubted. The rest of 
the ectoderm in Fig. 83 is composed of flat, thin cells, but the 
area in question has precisely the same appearance as in the 
swimming larva. The long spicules too have remained about 
in the same position which they occupy in the swimming 
larva, pieces of them being shown in the figure directly 
beneath the deeply stained patch of ectoderm. The variation 
which the Tedania larvae exhibit in their manner of attachment, 
is shown in other silicious sponges (see section on Morphology 
of Sponges, p. 364). 
That the columnar ectoderm of the larva flattens and is not 
cast off, is evidenced by the fact that during the metamor- 
phosis the sponge retains its smooth surface, and that no mem- 
brane or bit of membrane is seen to be sloughed off. The 
flattening of the ectoderm takes place quickly, being completed 
a very short time after the fixation of the larva. In Fig. 83 
the flattening of the columnar cells has taken place. After 
the polar ectoderm (ec. uz-f. ~.) has in like manner flattened, 
the entire investing layer of cells is so thin that it is best 
described as a nucleated membrane, ec., Pl. XXI, Fig. 89. 
During the flattening of the ectoderm the parenchyma of 
the larva also undergoes changes, as may be seen on compar- 
ing Figs. 81 and 83. In the attached sponge there are two 
kinds of cells which have no definite arrangement. There are 
first, great numbers of very small cells, so small that only the 
nuclei are seen with distinctness, the cell outlines being 
practicably indistinguishable ; and there are also numbers of 
deeply staining, plump-bodied, granular cells, such as were 
found in the spicular end of the swimming larva. The sponge 
shown in Fig. 83 (vertical section) flattens out considerably, 
especially at its periphery, and assumes the shape indicated by 
the vertical section, Fig. 92, and the surface view, Fig. 84, the 
parenchyma remaining practically unchanged. With regard 
to the rearrangement of the spicules of the swimming larva, 
something has already been said of the long (tylote) spicules. 
