334 WILSON. [Vol. IX. 



ectodermal area (cc. iin-f. /.) 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 fi.xation of the larva. In Fig. 83 

 the flattening of the columnar cells has taken place. After 

 the polar ectoderm {cc. un-p. p.) has in like manner flattened, 

 the entire investing layer of cells is so thin that it is best 

 described as a nucleated membrane, cc, PI. 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. 



