41 



may be aggregated on a simple or branching stem, or 

 immersed within a mucilaginous matrix. Excepting that 

 the social monads belonging to the last-named category 

 project directly upon the external surface of this common 

 mucilaginous matrix, instead, of indirectly upon chambers 

 with canal communications excavated in its substance, they 

 are to all intents and purposes a kind of sponge, most 

 nearly allied to that group possessing no spicular elements 

 already referred to under the title of the slime sponges or 

 Myxospongiae. In addition to the flagellated cells that 

 line the excavated chambers, other cell elements, devoid of 

 collars or flagella, and having an irregular shape, which 

 they are, moreover, continually changing, will be found 

 dispersed among the mucilaginous matrix of the sponge 

 body. In the fresh-water type, under discussion, these 

 irregular-shaped so-called amoebiform cells, are somewhat 

 thinly scattered throughout this matrix, but in many other 

 types, notably the Calcareous Sponges, they are so crowded 

 together, especially on the external surface, as to closely 

 resemble a veritable cellular tissue, and have been con- 

 sequently likened to the integumentary layer or epiderm 

 of the higher animals. The flagellated cells with their 

 hyaline collars that line the internal chambers of the 

 sponge, have in a like manner been compared to the 

 endoderm or lining tissue of the alimentary track of such 

 animals. The motile reproductive bodies liberated from 

 adult sponges, and that ultimately settling down, grow 

 into a sponge-stock like the parent, have likewise been 

 described as possessing similar external and internal cellular 

 membranes. It may, on the other hand, be demonstrated 

 that the non-flagellate amoebiform elements in the sponge 

 body are merely metamorphosed phases of the flagellated 

 cells, differing in no way from the various metaniorphic 



