SPONGES. 



53 



the passages of the drainage system, which should be considered as the endoderm. To' 

 this latter system the ampullae belong, but the endoderm which lines them is of a diJEfer- 

 ent character. The tubes of the supply system are doubtless of ectodermic origin. The 

 endodermal cells are usually flat 



and have polygonal outlines, 

 except in the ampullae, where 

 they give place to oval or even 

 columnar cells, the free ends 

 being crowned by transparent 

 collars, from the centre of 

 which protrudes a long flagel- 

 lum (Fig. 52). These collared 

 cells have unusually large nuclei. 

 The ectodermal cells vary some- 

 what in outline, according to 

 position, but are usually hex- 

 agonal or quadrangular and 

 rather constant in form. The 

 cells of the endoderm, on the 

 contrary, are subject to extra- 

 ordinary changes, btilging out 

 into balls on their free side 

 when gorged with food, or ex- 

 tending to hair-like cells of en- 

 ormous length when stretched 

 across an opening. 



Between these two layers 

 lies the middle or fleshy layer 

 of the body, the mesoderm. 

 This is composed of cells, but 

 the intercellular spaces are so 

 abundantly filled with proto- 

 plasm that Haeckel and others 

 consider it as a characteristic of 

 the sponges. We are, however, 

 of the opinion that the abun- 

 dance of intra-cellular substance 

 has been greatly exaggerated, 

 and that the mesodermal cells 

 are numerous and closely ag- 

 gregated. Such we have found 

 to be the case with the Calci- 

 spongiag and Chalina, and Lie- 

 berkuhn and Huxley claim the 

 same for Spongilla. The cells 

 of the mesoderm vary considerably in character and appearance. They may be 

 transparent, granular or deeply colored, globular or elongated, entire or amoeboid 

 in outline, and capable of extensive changes by expansion or contraction. In many 



Fig. 49. — Section of Halisarca, showing supply (a/) and drainage (c/) 

 systems, the ampuUse iflmp), and eggs in various stages of devel- 

 opment (a, 6, c, d, e,f). 



