i88 



PORIFERA 



inhalant canals, between them ; and where the walls of adjacent 

 chambers come in contact, fusion may take place. Pores guarded 

 by porocytes put the inhalant canals into communication with 

 the flagellated chambers. The paragaster is lined by pinaco- 

 cytes; choanocytes are confined to the flagellated chambers. 



The skeleton is partly defensive, partly supporting ; one set 

 of spicules strengthens the walls of the radial tubes and forms 



collectively the " tubar skeleton." 

 It is characteristic of Sycettidae 

 that the tubar skeleton is of the 

 type known as " articulate " — i.e. 

 it is formed of a number of 

 successive rings of spicules, in- 

 stead of consisting of a single 

 ring of large spicules which run 

 the whole length of the tube. 



The walls of the paragaster 

 are known as the"gastral cortex"; 

 they contain quadriradiate spi- 

 cules, of which the triradiate 

 systems lie tangentially in the 

 gastral cortex, while the apical 

 ray projects into the paragaster, 

 and is no doubt defensive. The 

 distal ends of the chambers 

 ^^°-^'^--Sy<'onsaosum Young Sponge bristle with tufts of oxeate 



X 200. a, Dermal cell ; g, gastral ■ t -i ^ 



cell ; 0, oscuium ; p, pore cell ; s^jj, spicules, and the separate cham- 

 r,?''°,T ' s^" *"™''^^*' 'P^™i"- bers are distinguishable in sur- 



(After Maas.) _ o 



face view. It is interesting to 

 notice that in some species of Sycon, the gaps between the 

 distal ends of the chambers are covered over by a delicate 

 perforated membrane, thus leading on, as we shall see presently, 

 to the next stage of advance.-' The larva of Sycon is an 

 amphiblastula (see p. 227). Fig. 80 is a drawing of the young 

 sponge soon after fixation ; it would pass equally well for an 

 ideally simple Ascon or, neglecting the arrangement of , the 

 spicules, for an isolated radial tube of Sycon. Figs. 81, 82 show 

 the same sponge, somewhat older. From them it is seen that 

 the Sycon type is produced from the young individual, in what 

 1 Dendy, Quart. Journ. Micr. Sci. xxxv. 1894, p. 230. 



