162 ARTHUR DENDY. 



number of the species referred to, or references to places where 

 they are to be found, are given in my " Synopsis of the 

 Australian Calcarea Heteroccela" (4). 



Melbourne ; 



January, 1893. 



II. The Canal System of the Calcarea Heteroccela. 



The simplest type of canal system in the group is found in 

 the genus Sycetta. This is, unfortunately, a genus which I 

 have never had the opportunity of personally examining, and I 

 am indebted for my information concerning it to Haeckel's 

 great work on the Calcarea (5). 



A Sycetta individual consists, in the first place, of a central 

 tube, which bears at its summit a single osculum leading from 

 the cavity of the tube (gastral cavity) to the exterior. This 

 central tube gives off all around and throughout its length 

 numerous short, hollow, conical diverticula (the radial flagel- 

 lated chambers). Each radial chamber communicates by a 

 single exhalant opening at its proximal end with the central 

 gastral cavity, and each has its wall perforated by numerous 

 much smaller apertures (the prosopyles), through which the 

 water passes from the exterior into the cavity of the chamber. 

 It is important to notice that the radial chambers, of which 

 there may be a large number, all stand perfectly free from one 

 another and do not touch at any point, so that the water is 

 free to circulate between them without obstruction of any kind, 

 and to penetrate right in to the outer surface of the wall of the 

 central gastral cavity. Hence there is no true inhalant canal 

 system, further than the small prosopyles by which the water 

 gains access to the interior of each chamber. The wall of the 

 central gastral cavity is very thin, so that the chambers open 

 almost directly into the latter, the wide " exhalant canals " 

 which conduct the water through the wall being comparatively 

 short and inconspicuous. 



The collared cells are, of course, confined to the interior of 

 the radial chambers, while the central gastral cavity and the 



