No. 3-] DEVELOPMEXT OF MARIXE SPOXGES. 309 



surface, os., PL XVIII, Figs. 4S, 51, 52, at the extreme periphery, 

 PI. XVII, Fig. 45, and PL XVIII, P'ig. 52, and even on the under 

 surface, PL XVIII, F"igs. 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. 



Ajfcrent Canals and P ores . — 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. d. 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 have a 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 can." 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 (J>. c, Fig. 57), an 

 instance of which is shown in Fig. ^'^,p-for. The peripheral 

 zone is perforated from one surface to the other by an aper- 



