E. 3IAESHALLI. — GEN. AEKAXGEMEXT OF SOFT PAFvTS. 117 



themselves only in the form either of cobweb-like or film-like 

 trabeeul?e or of a cribellate membrane (chamber-wall, raembrana 

 reticularis). All the soft parts, if put together apait by them- 

 selves, would make up but a comparative]}' small volume falling 

 considerably below the mass of the spicules and would appear 

 almost insignificant in proportion to the space occupied by the 

 entire sponge in its undisturbed state. 



General arrangement of the soft parts and their 

 RELATION TO THE SPICULES. — The flagellated chambers, whose 

 structure will be specially dealt with in the next paragraph, are, 

 as is well known, arranged side by side in a single layer, the 

 chamber-layer (PI. IV, fig. 28, ch. I.), which separates the outer 

 from the inner trabecular layer of the sponge-wall (see p. 41). 

 As is further known, the chamber-layer (which is not to be 

 confounded with the chamber-wall) forms in the choanosorae 

 numerous, outwardly directed protuberances or evaginations, 

 which are proximally open and distally blindly closed. The 

 evaginations aie of various sizes and of great complexity of form. 

 While some are quite small and simple, others, especially those 

 that extend into the parietal ledge and correspond in position with 

 the larger excurrent canals, may be of very considerable length 

 and caliber, and moreover bear on their sides a greater or less 

 number of secondary evaginations, which may again repeat the 

 branching process. The final branches belonging to diflereut 

 systems of the evaginations remain separate, although the possi- 

 bilitv of their coming into an intercommunicating anastomosis 

 under exceptional circumstances can not be excluded. 



The chamber-layer may then be considered as forming by 

 itself a voluminous mass with its complex system of evaginated 



