STUDIES ON THE COMPARATIVE ANATOMY OV SPONGES. 167 



left with three species, all of which exhibit the same type of 

 canal system, in which the radial chambers are short, straight, 

 and unbranched, and project quite separately and indepen- 

 dently from the wall of the central gastral cavity. 



According to Haeckel (5) there is in Sycetta primitiva, 

 and also in certain more highly organised species of Syconoid 

 Heterocoela, a single larger aperture at the distal end of each 

 radial chamber, which he terms a " dermal ostium .'■' PolejaeflP, 

 however, has (8) thrown considerable doubt on the existence 

 of " dermal ostia " in any case, and I cannot help agreeing 

 with him in attributing the supposed presence of these 

 structures to an error of observation on the part of their 

 describer. 



Sycon (figs. 2 — 7). 



In this genus we meet with a considerable amount of varia- 

 tion m the canal system. The simplest form is found in such 

 species as the European S.ciliatum (Bauerbank^s Grrantia 

 ciliata) and S. raphanus, and in the Australian S. Carteri 

 and S. minutum (4). For a detailed account of S. raphanus 

 1 may refer the student to Schulze's well-known and admi- 

 rable memoir (7). 



These species mark but a slight step upwards from the 

 Sycetta type. In Sycon Carteri, for example (fig. 2), the 

 radial chambers are rather short, and more or less thimble- 

 shaped; they touch one another in some places, and there fuse 

 together by their outer surfaces. Their distal ends, however, 

 project freely, and thus form well-marked " distal cones " 

 on the surface ot the sponge. By the fusion of adjacent 

 chambers at the points of contact, the originally continuous 

 water-containing space which surrounds all the chambers 

 becomes broken up into more or less detinite inhalant canals 

 ("inter-canals"), whose exact form depends, of course, upon the 

 shape of the chambers and the extent to which they fuse to- 

 gether, and is of no great importance, in the simplest cases, such 

 as Sycon Carteri (tig. 2) these canals appear, in sections taken 

 along the length of the chambers, as straight narrow gaps 



