SPONGES. 



53 



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

 this latter system the ampullai belong, but the endoderm which lines them is of a differ- 

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

 ciidodermal cells are usually flat 

 and have polygonal outlines, 

 except in the ampullte, where 

 they give place to oval or even 

 columnar cells, the free ends 

 Ix'ing crowned by transjiarent 

 collars, from the centre of 

 uhich protrudes a long flagel- 

 him (Fig. 52). These collared 

 (•ells have unusually large nuclei. 

 The ectodermal cells vary some- 

 what in outline, according to 

 jiosition, but are usually hex- 

 agonal or quadrangular and 

 rather constant in form. The 

 cells of the endoderm, on the 

 contrary, arc subject to extra- 

 ordinary changes, bulging out 

 into balls on their free side 

 ulien 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. 



Tliis is composed of cells, but 



the intercellnlar spaces are so 



abimdantly filled with proto- 

 plasm that Haeckel and others 



consider it as a characteristic of 



the sponges. AVe are, however, 



I if the opinion that the abun- 



diiiice of intra-cellular substance 



has been greatly exaggerated, 



and that the mesodermal cells 



arc numerous and closely ag- 

 gregated. Such we have fouiid 



to 1)0 the case with the Calci- 



spongiaj and C/ialina, and Lie- 



berkuhn and Huxley claim the 



same for Sponfjillu. The cells 



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



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



in outhne, and capable of extensive changes by expansion or contra<.tion. In many 



Fig. 49 —Section of Halisarca, showing supply («/•) and drainaae (ef) 



