120 



de même; mais pour aller des premières aux secondes, on se 

 lieurterait partout a une cloison de choanosome. Dans ces cloisons 

 sont les corboilles, petites, arrondies, dépourvues de prosodus et 

 d'apliodus, s'ouvrant d'une part dans les lacunes inlialantes par 

 deux a cinq petits orifices prosopylaires et d'autre part dans les 

 lacunes exhalantes par un large orifice apopylaire, ce qui permet 

 de distinguer sur les coupes les deux sortes de lacunes. Dans ces 

 cloisons sont aussi, entre les autres elements mésodermiques .... 



les spicules formant dans Ie réseau du choanosome un 



réseau squelettique Ces spicules sont soudés entre eux, soit 



dans toute leur longeur, soit par leurs extrémités seulement, par 

 la spongine". 



Delage and Hérouard's illustration with few alterations 

 (Fig. 53) gives rather a good idea of the canal-system ; it is, how- 

 ever, but a diagram. A better illustration of the surroundings of 

 the flagellated chambers in Spongillidae is given by Fig. 54. 



Finally a beautiful illustration of a flagellated chamber of Spon- 

 gilla lacustris (Fig. 55) is taken from Vosmaer and Pekelharing 

 (61). The authors mention that this figure was drawn with great 

 care from a very carefully preserved preparation. The apopyle 

 (ap.) and the excurrent canal is shown. The figure represents a 

 section of a chamber; therefore we see but one flagellum at 

 its real length. The thin line uniting the bases of the choano- 

 cytes represents the outline of the chamber. So we see that the 

 chamber is lined by the well known choanocytes, whose collar 

 and flagellum can be easily distinguished in the figure as well 

 as their nucleus, a vacuole (at the base of the flagellum) and a 

 certain number of black spots, which are doubtless the above men- 

 tioned (p. 90, 9) oildrops. Finally I want to mention that the 

 choanocytes are able to entirely retract their collar and perhaps 

 also their flagellum. 



Function. — Proceeding to the description of the water-cur- 

 rent in the canal-system, I want to remind in the first place 

 that the water enters the inhalant (incurrent) system through the 

 ostia, it then passes from the incurrent canals trough the proso- 



