PORIFERA. 



217 



Amongst the calcareous Sponges, the simple Sponge with inhalent 



pores and terminal osculum (Olynthus-iorm) is represented by the 



stock-forming Leucosolenia (Grantia), which is composed of numerous 



hollow cylinders. The structure of this sponge 



has been described by Lieberkiihn. 



In the Syconidw the body cavity has a more 



complicated form. The central space opens into 



secondary peripheral spaces or radial tubes, which 



are lined by ciliated cells, and open externally 



through the inhalent pores (fig. 162). 



In other calcareous Sponges (Leuconidce) the 



radial canals have the form of irregular parietal 



canals, giving off branches to the periphery and 



possessing dilated, ciliated chambers. This form 



of internal canal system is also found in most 



of the stock-forming, silicious Sponges (fig. 163). 

 Sponge forms may become more complicated 



by the formation of stocks ; the originally simple 



Sponge, which has developed from a single cili- 

 ated larva, gives rise by budding and incomplete 



fission to a polyzoid sponge body; or several 



originally 

 separate 

 iriclividu - 

 als, each 

 of which 

 has origi- 

 nated from a single larva, 

 fuse together to form a com- 

 pound sponge stock. Both 

 these methods of growth are 

 repeated in a similar manner 

 in the formation of the stocks 

 of Polyps (fig. 164). In the 

 same way that the fan-like 



FIG. 1G2. Longitudinal 

 section through Sycon 

 raphanug, slightly mag- 

 nified. 0, Osculum 

 with collar of spicules; 

 Jit, radial tubes which 

 open into the central 

 cavity. 



FlO. 103. Section of Cort icium candelabrum (after nets of the Fan Coral 



th dogorgia flaMUm) are formed 

 by the repeated fusion of its 

 branches, the gastrovascular cavities of which anastomose, so also 

 in the case of the branching sponges, as a result of the same pro- 

 cess, reticulate, or coiled or even massive stocks are formed (fig. 165). 



