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 'J pr.fi "^'^ ) > 



tubes, which become closed 



in to form definite incurrent ) C^ "'' j> 



canals. In the simplest Vy^"-- --*'---"- h---jl__-^_^--..^ q^ 



case (Fig. 67) a dermal est --■* uv.c } » 



membrane is formed by A ---^.,^, — _-„ ,..-.^^^- ^--^^ 



outgrowths from the ex- r.-C l' 



tremities of the radial \^--^ 'w J^ r . j w la i j e^::^-.^ 



tubes, in exactly the same ' r.v>i 



way as in the formation Fro. or. 



of a pseudoderm in AsCOns, section of the body wall of Sycon gdatlnosum. The 



1,1 , , ,1 external surface is to tlie left, the internal surface to 



and tne entrance to tne the ri;,'ht. in.c, incurrent canal ; pr.p, prosopyle ; r.t, 



inr^iirrpnf ci-i'ipa ic tlino radial tube (flagellated chamber); app, apopyle ; o»<, 



lULUiieni, bpace I^5^ luus ostium ; C.C, gastral cavity. 



narrowed to a circular 



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

 pore of Clathrina ventricosa. The incurrent space becomes furrier 

 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. Finall}^ 

 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 Oli/nthus 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 Avith 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;. 



