Spongice or Porifera. 



119 



In the simplest form (Fig. 74 A), the body, which is always 

 attached to some foreign object, consists of a sac open at the upper 

 end, and composed of three layers. Of these, the 

 ectoderm consists of flattened cells, each furnished 

 with a flagellum ; the middle layer, of a mass of con- 

 nective tissue ; and the endoderm, again, of a special 

 form of flagellate cell (Fig. 73), peculiar in bearing 

 at the free end a thin drainpipe-like tube, within 

 which is the flagellum (collar cells). The cavity 

 of the sponge (exhalent canal) not only com- 

 municates with the exterior by the large opening 

 (o sculum), but also byinhalent canals, which 

 perforate the wall and open by p o r e s. The inhalent 

 current of water passes through the pores into the 

 large cavity, and out through the osculum ; the 

 movement is kept up by the flagella. 



The simplest of these types occurs only in a minority of the 

 Sponges (in certain calcareous forms) . In others, it is complicated by 



Fig. 73. Collar 

 cells of aSponge. 

 fc collar. 



Fig. 74. Various forms of Sponges; diagrammatic longitudinal sections, o osculum, 

 p pores. — Orig. 



saccular evaginations of the exhalent canal (Fig. 74 B). The 

 lining of collar cells is limited to these chambers, whilst the main 

 cavity is lined by a flat epithelium ; the inhalent canals open into the- 

 evaginations. In others (Fig. 74 C, the left side of the figure), the 

 evaginations are, again, provided with smaller pouches, and in these 

 alone, collar cells are found; they are, therefore, termed the 

 flagellate chambers; they communicate with the exterior by 

 branching inhalent canals. Finally, the chambers may be racemose, 

 that is, they may be connected with the main branches by longer or 

 shorter stalks (Fig. 74 C, the right side of the figure) . In all cases,. 



