PORIFERA. 



some part of the canal system, whence they are carried to the 

 exterior. 



It is probable that all the protoplasmic tissues of the sponge are 

 contractile, i.e., both the epithelial layers and the mesodermal net- 

 work ; but special structures in the course of the inhalent and 

 exhalent parts of the canal system have been described as muscular 

 sphincters. Ectodermal cells carrying hair-like sensory projections 

 have been described ; these, like other epithelial structures, are 

 connected below with the mesodermal network, parts of which 

 have therefore been interpreted as nerve-fibres and nerve-cells ; but 



Fio. 63. Piece of network of 

 horny fibres from Euspongia 

 equina. 



Mes 



FIG. 64. Section through a calcareous sponge (Sycon 

 mphamts), after F. E. Sch. Ect ectoderm; En 

 endoderm of a flagellated chamber ; Mes meso- 

 derm ; N calcareous spicule in the mesoderm ; 

 Eiz ovum. 



there is no reason why one part of the network should be considered 

 as more especially adapted for nervous conduction and reflection than 

 another. The ectodermal epithelium consists of flat cells; the 

 endoderm is partly formed of flat cells and partly of somewhat 

 cylindrical cells, each with a flagellum and collar. These are the 

 choanocytes (Fig. 64). They are perhaps the most characteristic 

 constituents of the sponge-body ; the collar is a membranous pro- 

 longation of the cell at its free end round the base of the flagellum ; 

 and the whole cell resembles an individual of the Choanoflasjellata. 

 It contains one or more contractile vacuoles, and its base is prolonged 



