10 ART. 5. — SAN.JI HüZAWA : 



central gastral cavity, extending nearly through the entire thick- 

 ness of the sponge wall. They are broad in the proximal parts 

 and become markedly narrower in the distal parts. Sometimes 

 they are seen to be united with one another in the proximal parts 

 and also to divide distally into branches. The nucleus of collar 

 cells occupies an apical position. 



The gastral cortex is very thin with a small quantity of 

 mesoglœa ; it is pierced by very short exhalant canals, arising 

 from either a single or fi-om 2-4 flagellated chambers. A well- 

 developed diaphragm exists at the apopyle. The exhalant canals 

 open into the gastral cavity through small angular meshes, meas- 

 uring 200-400 /^ across and formed by intercrossing rays of gastral 

 tri- and quadriradiates. 



Skeleton (PL n., fig. 12). 



The dermal skeleton is made up of : 1) triradiates tangential - 

 ly disposed in a few layers, with basal ray generally directed 

 downwards ; 2) the paired rays of subdermal pseudosagittal tri- 

 radiates ; 3) large oxea, which occur here and there in vertical 

 disposition in the sponge wall and which usually project outwards 

 on the dermal side only but may sometimes do so also on the 

 gastral side ; 4) trichoxea which are but rarely found. 



The tubar skeleton is composed of : 1) the centripetal basal 

 rays of subdermal pseudosagittal triradiates ; 2) the centrifugal 

 basal rays of subgastral triradiates ; and 3) one or two intermediate 

 rows of triradiates which, have their basal ray directed outwards. 

 The above refers to the thick-walled middle parts of the sponge. 

 In the parts close to osculum where the wall is thin, the tubar 

 skeleton consists only of the basal rays of subdermal and sub- 

 gastral triradiates. 



