No. 3.] DEVELOPMENT OF MARINE SPONGES. 28 1 



(the so-called subdermal cavities, j-.(/.r., PI. XIV, Fig. 2), separates 

 more or less completely the superficial layer of the sponge from 

 the rest of the body. This superficial layer (ectoderm + thin 

 layer of mesoderm) is known as the dermal membrane, d. 7ncvi., 

 Fig. 2. The dermal membrane is bound to the subjacent part 

 of the body by the mesoderm lying between the subdermal 

 cavities. The cavities are so numerous that this mesoderm 

 takes the form of irregular beams or trabeculae, often being 

 nothing more than slender cords, as in Fig. 4, vies. b. The 

 dermal membrane may easily be pulled off, and is then found 

 to be everywhere perforated by closely set pores, Fig. 4 (surface 

 view of dermal membrane, from below). The subdermal cavities 

 into which the pores open are of two kinds, of which the larger 

 and deeper appear as pore areas (often rounded) on the surface. 

 These pore areas are conspicuous when the surface of the 

 sponge is examined with a lens, and the dermal membrane 

 between them seems at first sight not to possess any pores 

 (Fig. 5, view of a small area of the surface). This is owing to 

 the fact that the second kind of subdermal cavity, underlying 

 the apparently aporous portion of the dermal membrane, is very 

 small and shallow. That the dermal membrane does possess 

 pores between the pore areas can easily be shown by scraping 

 the membrane free of the subjacent tissue, when the pores at 

 once come into view (region a in Fig. 5). Owing to the great 

 number of pore areas the surface of the sponge acquires a 

 characteristically porous appearance. When examined under a 

 low objective or magnifying glass the surface is further diver- 

 sified by an irregular meshwork of dark bands, which represent 

 the coarser mesodermic trabeculae connecting the dermal mem- 

 brane with the body of the sponge (Fig. 5). 



The water passing through the pores enters the subdermal 

 cavities, s.d.c, Fig. 2, whence it may pass by afferent canals, 

 a.f. c, to the flagellated chambers. The flagellated chambers 

 are arranged along the sides of the afferent canals, and 

 open directly into them. At their opposite pole the 

 chambers open in the same way into efferent canals {ef. c). 

 Both afferent and efferent canals are relatively large. There 

 is no great difference in size between the afferent canals 



