STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 31 



halfway. The migrations of the ova in sponges remind one 

 forcibly of Weissmanu's descriptions of the migrations of the 

 ova in various hydroids (25). 



The Post-embryonic Development of Grantia 



labyrinthica. 

 The embryology of Grantia labyrinthica^ so far as the 

 material at my disposal has allowed me to work it out, forms 

 the subject of a special memoir (10), so that it is unnecessary 

 for me to deal with the question in this place. The post- 

 embryonic development, however, has not as yet been dealt 

 with, and although there is but little to be said about it, yet 

 that little is of interest as clearly indicating the manner in 

 which the very peculiar external form has been evolved. 



Fig. 1 shows the youngest stage found, in which it will be 

 seen that, as regards both size and shape, the sponge diflfers 

 but little from an ordinary Grantia. The wall of the tube 

 is even and not convoluted, although already the osculum is 

 the widest part of the gastral cavity. Fig. 27 represents 

 somewhat diagrammatically a longitudinal section of this 

 specimen, and is important chiefly because it shows how the 

 stalk arises by the filling up of the lower portion of the gastral 

 cavity with a copious growth of mesodermal tissue in which 

 numerous spicules are developed. As I have already pointed 

 out, remnants of the gastral cavity may sometimes be recog- 

 nised in the stem even of adult specimens. Figs. 2 and 3 

 represent two stages intermediate between that just described 

 and the full-grown sponge (fig. 4). The specimen drawn in 

 fig. 3 is very much compressed, so as to be bilaterally sym- 

 metrical. This may either be accidental, or it may indicate 

 the commencement of folding in the wall of the cup. All 

 figures are drawn of the natural size, and they render further 

 description needless. 



I may conveniently here describe the only case of budding 

 with which I have met in the species. On the outer surface 

 of the wall of the cup in a well-grown specimen, and not 

 far below the margin, a small individual had budded oflF 



