3o8 wilsoa: [\'ol. ix. 



PL X\'II, 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, PI. 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 PI. XVII, Figs. 42, 44 and 50. Scattered 

 amongst the bipolar cells are quite a number of larger rounded or 

 branched (formative) cells, PI. 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 {cf. c, in PL XVII, Fig. 50, is very probably 

 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 



