168 ARTHUR DENDY. 



between adjacent chambers. In the case of Sycon raphanus, 

 Schulze (7) has shown that the different inhalant canals often 

 inter-communicate through gaps left by the incomplete fusion 

 of adjacent chambers, and this condition probably occurs to a 

 greater or less extent in a great many cases. 



Probably in most species of the genus Sycon, and certainly 

 in the four with which we are now more immediately con- 

 cerned, the distal ends of the inhalant canals ("inter-canals ") 

 are in no way closed in ; in other words, the spaces between 

 the distal cones of the radial chambers remain widely open. 

 Thus the water still has direct access to the prosopyles, with- 

 out having to pass through the pores of a dermal membrane. 

 In Sycon ensiferum (4), a species closely related to S. 

 raphanus, the basal rays of many of the most distally 

 situated tuber triradiates are very strongly bent outwards from 

 the walls of the chambers, so as to curve over and protect the 

 entrances to the inhalant canals. 



In the common Australian S. gelatinosum (figs. 3 — 6) 

 we meet with a slight but very interesting advance in organi- 

 sation. The radial chambers, though of considerable length, 

 are still straight, and usually unbranched, subcylindrical 

 tubes. The mesoderm of the chamber walls is strongly de- 

 veloped, and the inhalant canals between the chambers are 

 very well defined and squarish in transverse section (fig. 8). 

 The gastral cortex is rather thick, and consequently the 

 exhalant canals take the form of distinct, though short 

 and wide, tubes, sharply marked off from their respective 

 chambers by well-developed chamber diaphragms, which 

 appear to be almost universally present in calcareous sponges 

 (fig. 4). A tangential section of the dermal surface (fig. 6) 

 shows that the distal ends of the chambers, each crowned with 

 its tuft of oxeote spicules, do not touch one another, but are 

 separated by rather narrow gaps or spaces, across which 

 stretches a delicate pore-bearing membrane (seen in section in 

 fig. 3). This membrane is doubtless formed as an outgrowth 

 of the ectoderm and mesoderm of the walls of the chambers. 

 In Sycon gelatinosum it contains no spicules. The fusion 



