SPONGES 97 



which are now better distinguished as the incurrent or inhalant 

 system. 



The further development of the syconoid type takes place chiefly 

 by a narrowing of the 

 primitively wide incurrent 

 spaces between the radial 

 tubes, which become closed 

 in to form definite incurrent 

 canals. In the simplest 

 case (Fig. 67) a dermal ost 

 membrane is formed by 

 outgrowths from the ex- 

 tremities of the radial 

 tubes, in exactly the same 

 way as in the formation FIG. or. 



of a pseudoderm in AsCOnS, Section of the body wall of Sycon gdatinosum. The 



i , , 4- fV, external surface is to the left, the internal surface to 



anu. tne entrance tO tne the right, in.c, incurrent canal ; pr.p, prosopyle ; r.t, 



incurrent SDace is thus ra( l ial tube (flagellated chamber) ; app, apopyle ; ost, 



ostium ; G.C, ga.stral cavity. 



narrowed to a circular 



aperture, the dermal pore or ostium (ost), comparable to a pseudo- 

 pore of Clathrina ventricosa. The incurrent space becomes further 

 reduced by coalescence taking place between adjacent radial 

 tubes where they come into contact, thus interposing partitions, as 

 it were, which divide up the continuous incurrent space. Finally, 

 in many forms the dermal layer at the distal extremities of the 

 radial tubes becomes thickened to form a cortex, through which the 

 narrow incurrent canals pass to reach the radial tubes (Figs. 68, 

 69). These changes, and especially the formation of a cortex, have 

 the effect of completely masking the folded and lobed appearance 

 of the body wall, which results from the outgrowth of the radial 

 tubes, and the outer surface of the body presents a smooth, porous 

 surface, so that the form and appearance of the Olynthus may be 

 perfectly retained (Figs. 9, 10). 



In addition to these changes in the incurrent system, various 

 modifications may take place in the radial tubes, or in their relations 

 to the gastral cavity. In the first place, the radial tubes may become 

 very much branched and secondarily complicated. A more im- 

 portant change, however, from the morphological point of view, is 

 the formation of an excurrent duct connecting the radial tube with 

 the gastral cavity that is to say, the flagellated chamber is, as it 

 were, carried outwards, and does not open into the gastral cavity 

 directly, but communicates with it by means of a short duct lined 

 by flattened epithelium. At the same time the excurrent aperture, 

 or apopyle, of the chambers may become greatly contracted, appear- 

 ing as a perforation in a diaphragm separating the chamber from 

 its excurrent duct (cf. Fig. 67). 



