PORIFERA lOI 



the mesogloea, into which some cells cast fluid or solid substances 

 which are taken up by other cells. 



All waste ^ such as undigested material, etc., is carried out 

 through the osculum. Exchange of waste gases for oxygen is 

 also effected by this current of w^ater. 



Around the outer edges of the incurrent and excurrent canals 

 and the osculum are elongate cells (ectodermal and endodermal) 

 which are contractile and thus represent muscles. These 

 muscle cells effect the closing of these openings. 



There is probably in all sponges a total absence of special 

 nerve cells, and hence a great lack of coordination in cell move- 

 ments, " thus the flagella of the collar-cells do not beat in uni- 

 son like the cilia of the epithelia in higher animals, but each 

 works independently of the others " (17). 



Reproduction takes place through the development of the 

 sexual reproductive cells, — spermatozoa and ova ; these develop 

 from the amoeboid wandering cells of the mesog^loea, which for 

 this take up their position directly below the flagellate cells. 

 Both ova and spermatozoa are developed in the same sponge 

 individual, though rarely at the same time. The amoeboid 

 cell, to form spermatozoa, divides into many small cells, each 

 of which develops a long tapering tail. The amoeboid cell, to 

 become an ovum, simply becomes round and enlarges, and after 

 a sperm has penetrated it, thus effecting impregnation, the ovum 

 becomes inclosed in a brood-pouch made by the neighboring 

 cells. In its development the ovum, after its impregnation, 

 divides into two cells ; these divide again and again, resulting in 

 a hollow sphere, the blastula. In this shape, the embryo es- 

 capes to the exterior and moves about by means of flagella. 

 Later the layer of cells from one side bends in against the layer 

 of the opposite side, forming a double-walled cup, the gastrula. 

 The opening of this gastrula cup, the blastopore, gradually closes, 

 and at this stage of its development the young sponge becomes 

 fixed by the side on which was situated the blastopore ; an open- 

 ing, the osculum, breaks through at the opposite end and the 



