. The Cambridge natural history. Zoology. 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 ; It is characteristic of Sycettidae that the tubar skeleton is of
. The Cambridge natural history. Zoology. 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 ; It is characteristic of Sycettidae that the tubar skeleton is of the type known as " articulate "â 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 â t -i ^ cell; 0, oscuium; p, pore cell; s^jj, spicules, and the separate cham- r,?''°,T' s^" *"â¢''^^*' 'P^â¢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 ' 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 f
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