CALCAREA HETEROCOELA 



189 



may be called its Ascon stage, by a process of outgrowth of tubes 

 from its walls, followed by restriction of choanocytes to the 

 flagellated chambers. Minute observation has shown ^ that this 

 latter event is brought about by immigration of pinacocytes from 

 the exterior. These cells creep through the jelly of the dermal 



Fio. Sl.—S. setosum. Young Sponge, with one whorl of radial tubes. 0, Osculum ; p, 

 pore ; sj>y monaxon ; s}}^, quadriradiate spicule. (After Maas.) 



layer and line the paragaster as fast as its original covering of 

 choanocytes retreats into the newly formed chambers. 



With a canal system precisely similar to that of Sycon, Ute 



(Fig. 83) shows an advance in structure in the thickening of the 



dermal layers over the distal ends of the chambers. The dermal 



thickenings above neighbouring chambers extend laterally and 



1 Maas, Zeitsohr. wiss. Zool. Ixvii. 1899-1900, p. 215. 



