NOs. ] DEVELOPMENT OF MARINE SPONGES. 309 
surface, os., Pl. XVIII, Figs. 48, 51, 52, at the extreme periphery, 
Pl. XVII, Fig. 45, and Pl. XVIII, Fig. 52, and even on the under 
surface, Pl. XVIII, Figs. 52, 53. The appearance of oscula on 
the under surface is not of frequent occurrence, still I have found 
several in this position. The number and distribution of the 
oscula in these young sponges, as in the adult, is quite without 
regularity. There may be one or several, and they may be 
anywhere on the surface of the sponge. 
Afferent Canals and Pores. — The subdermal cavities in the 
young sponges act directly as afferent canals. The membrane 
above a cavity becomes perforated by pores, and in the floor de- 
velop flagellated chambers, Pl. XVII, Fig. 50, and Pl. XVIII, Fig. 
51. In Fig. 51, for instance, it is plain that the water entering 
into the subdermal cavity, s. @.c., must pass directly through the 
flagellated chambers in order to get into the efferent canal, ef. c. 
In the adult, many of the subdermal cavities havea floor made up 
of a layer of flagellated chambers (s. d. c’, Pl. XIV, Fig. 2), and 
in such cases it would look as if the water passed directly from 
the cavity into the chambers. Though the subdermal cavities 
undoubtedly act directly as afferent canals in the young sponge, 
other afferent canals are also developed. Some of the deeper 
lying cavities establish connection with the subdermal spaces, 
for instance caz."’ in Pl. XVII, Fig. 44, and these, it would 
seem, become afferent canals. 
In the sponges I reared, the number of pores that developed 
was not very great. In the oldest individuals each subdermal 
cavity had as a rule one or a few pores, Pl. XVIII, Figs. 58 and 
59. The pores developed as perforations, the edge being 
strengthened, as is true also of the edge of the oscula, by the 
presence of fibre-like mesoderm cells. In many cases a very fair 
number of pores had developed before an osculum made its ap- 
‘pearance. This is true of the sponge shown in Fig. 58. The 
distribution of the pores in the young sponge may be gathered 
without further description from the two Figs. 58 and 59. A 
curious phenomenon analogous to the formation of a pore 
sometimes takes place in the peripheral zone (f. z., Fig. 57), an 
instance of which is shown in Fig. 50, ~. for. The peripheral 
zone is perforated from one surface to the other by an aper- 
