308 WILSON. [VoL. IX. 
Pl. XVII, Figs. 44 and 45, the mesoderm cells of the dermal 
membrane still retain some trace of their former plump proto- 
plasmic body, but in stages yet older, Pl. XVIII, Figs. 48, 51, 
the cell body consists of a mere covering for the nucleus, con- 
tinued into, in the majority of cases, two long slender processes. 
The large number of such fibre-like cells converts the dermal 
membrane of the oldest stages reared into a tough, strong 
covering. In surface views the dermal membrane can best be 
studied over the subdermal cavities, where the slender bipolar 
cells of the middle layer of the membrane are very conspicuous 
(Pl. XVIII, Figs. 58 and 59, the latter representing a part of the 
peripheral region of a sponge like Fig. 58). The basal portion 
of the sponge undergoes a development somewhat similar to 
that of the upper crust. Many of the mesoderm cells in this 
region become transformed into bipolar or branched cells with 
very small bodies and slender long processes; compare the suc- 
cessive stages shown in Pl. XVII, Figs. 42,44 and 50. Scattered 
amongst the bipolar cells are quite a number of larger rounded or 
branched (formative) cells, Pl. XVII, Fig. 50 and Pl. XVIII, Fig. 
51. In this part of the sponge flagellated chambers are not de- 
veloped. To be sure while the canals are still few and wide 
apart, a few flagellated chambers may be found close to the basal 
ectoderm, Pl. XVII, Fig. 44, but after the system of canals 
becomes more extensive the basal portion of the sponge is no 
longer found to contain any chambers. The same is true of 
the dermal membrane, in which during the earlier stages there 
is occasionally (very rarely) found a chamber, Fig. 44, but 
which in later life is entirely devoid of such structures. 
Efferent Canals and Oscula. — Efferent canals are formed in 
this way. Some canal which usually extends deep into the tissue 
of the sponge (ef. ¢ in Ply XVII, Fig: 50; is very yprobably; 
going to develop into an efferent canal) breaks through to the 
exterior by a large opening, the canal becoming the efferent 
canal, the opening the osculum, Fig. 57. The osculum is pro- 
duced by simple perforation of the dermal membrane, the ecto- 
derm becoming continuous round the edge of the aperture with 
the lining of the canal. Oscula may be formed anywhere on 
the surface of the sponge, in the central region of the upper 
