PORIFERA. 



223- 



(p. 202), since they have collars surrounding the flagella. It has 

 therefore been attempted to regard each flagellate cell as an indi- 

 vidual, and the whole sponge as a colony of Flagellata, a view which 

 neglects the other tissues, not only the connective tissue and the 

 epithelium already mentioned, but sex cells, amoeboid wandering 

 cells, and contractile fibre cells which close the pores. The taking 

 of food is accomplished by the collared cells. 



Sponges of this simple ascon type are few. As a rule the 

 sponges are more massive and have a more complicated canal 

 system (figs. 164-166). The first step towards complication is 

 seen in the Sycon sponges (fig. 163), in which the gastral cavity/ 



FIG. 163. 



FIG. 164. 



FIG. 163. Stereogram of Sycon sponge (orig.). a, ampullae with pores in their walls;. 



c, cloacal chamber, with the openings of excurrent canals; i, incurrent canals; 



o, osculum. 

 FIG. 164. Section of Leucortis pulvinar. (After Haeckel.) a, aboral pole ; c, efferent 



canals from the ampullae to the cloaca; e, ampullae; t, mesoderm; o, osculum; v, 



cloaca. 



consists of numerous radial outpushings (the flagellate chambers 

 or ampulla) which alone contain the collared cells, while the cen- 

 tral cavity, now called cloaca, is here lined with pavement epi- 

 thelium. By increase of mesoderm and corresponding thickening 

 of the body wall the ampullae become separated from external and 

 cloacal surfaces by the ingrowth of tissue (Leucon type). The 

 ampullae nevertheless retain their connexion with both surfaces: 

 by means of a system of canals. . This canal system is double; one- 

 part is incurrent and leads from the dermal pores to the ampullae;; 



