No.3.] DEVELOPMENT OF MARINE SPONGES. 337 
by an ectodermal membrane). The change of contour con- 
cerns especially the parenchyma, which pushes out lobes and 
processes inside the ectodermal membrane, thus acquiring 
from time to time entirely different outlines, while the sur- 
rounding ectodermal membrane remains practically unchanged. 
If the parenchyma, however, continues to change its contour 
in such a way that the shape of the whole sponge is altered, 
as, for instance, in passing from a circular outline to a shape 
such as that in Fig. g1, then the ectodermal membrane is 
involved and its edge gradually altered so as to remain more 
or less parallel with the general contour of the parenchyma. 
In sponges which have assumed elongated irregular shapes, 
like that of Fig. 91, the change of contour sometimes leads 
to the complete division of the body into two independent 
sponges. This phenomenon I have twice observed. I thought 
at one time that I had witnessed the converse phenomenon, 
z.e. the fusion of two attached sponges into one. I observed 
two sponges, a couple of days after attachment, which lay near 
each other, grow nearer and nearer until after fifteen to twenty 
hours they met and seemed to fuse into one body of an irreg- 
ularly oval shape. Across this body, however, could be seen 
the seam or line of fusion, and the union must have been one 
of close juxtaposition only, for after a few hours the sponges 
again separated along this line and afterwards remained inde- 
pendent. Fusion of the swimming larvae, into a single large 
one, as occasionally happens in the Coelenterates (Manicina) 
I have never observed. 
Canal System.—The canals and subdermal cavities appear 
as separate lacunae in the parenchyma, the surrounding cells 
becoming modified into epithelioid membranes, Figs. 93 and 
94, s. d. c., can. The separate lacunae subsequently become 
united into a canal system, as in Esperella. The flagellated 
chambers likewise originate as independent structures, which 
later acquire connection with the canals, Fig. 93. The sub- 
dermal cavities, which in some cases are very extensive, as in 
Fig. 91, s.d.c., are roofed over by a dermal membrane (ad. mem. 
in Fig. 93), quite like the same structure in the young Es- 
perella. As in Esperella, no system is followed in regard to 
