1900] MICROSCOPICAL JOURNAL. 339 



bers have a radiate arrangement as in S. raphanus but 

 the ends do not project freely as in that form but are en- 

 closed in a cortex which is continuous from chamber to 

 chamber. The presence of this cortex necessitates some 

 change in the canal-system and we find special pore-areas 

 in it through which the water enters. The pores lead 

 into inhalent canals which communicate directly with the 

 prosopyles in the walls of the flagellate chambers through 

 which the water flows ; the chambers open by means of 

 very short exhalent canals in the gastral cavity, and the 

 stream of water passing out of each chamber is regulated 

 by the opening and closing of a diaphragm with which 

 each chamber is furnished. 



The last example of the group to be considered is one 

 named Vosmaeropsis macera. This sponge is something 

 like S. gelatinosum in appearance, but the tubes are more 

 densely agglomerated, the numerous individuals being 

 almost completely fused together. We take this form for 

 examination because of the modification of its canal sys- 

 tem as compared with the last example. Here the cham- 

 bers are thimble-shaped, but they are scattered somewhat 

 irregularly between the dermal cortex and the gastral 

 cavity, and as they lie at some distance from the gastral 

 cavity communication is effected by means of long exhal- 

 ent canals, whilst in S. gelatinosum these canals are short. 



The calcareous sponges described above do not, of 

 course, exhaust the list of the Flinders specimens. They 

 have been described in some detail because they show the 

 development of the canal-system from the simple type of 

 L. stolonifer to the more complex one of V. macera, and 

 they show it in an unmistakable manner. 



Reference has been made, again and again, to the pres- 

 ence of spicules. They lie in the mesodermal tissue of 

 the sponge, each being developed from a single cell, the 

 scleroblast, and are composed of carbonate of lime, and 

 give strength, support and protection to the softer tis- 



