of the Sponges to the Choanojlagellata. 371 



sometimes gelatinous, sometimes firm, and sometimes even of 

 cartilaginous hardness (corticium). Of these cells some are 

 freely movable, others fixed. The former can change their 

 place by amoeboid movements as " creeping cells j" while the 

 fixed ones are sometimes irregularly stellate in form, some- 

 times fusiform or even filamentous, and in many cases are 

 capable of well-marked contractions, and may sometimes even 

 resemble smooth muscular fibres in structure and function. 

 The basal substance as such, however , possesses no contractility. 

 It is not a sarcode or amalgamated cell-protoplasm, but an 

 intermediate substance distinct from the cell-bodies, sometimes 

 like that of the gelatinous connective tissue. This notion of 

 mine has lately been adopted by most spongiologists. 



Saville Kent further states, with regard to the gelatinous 

 foundation of his Protospongia Hceckelii, that, being at first 

 quite structureless, it becomes converted, by the immigration 

 of amoeboid individual animals from the surface, into a tissue 

 which exactly resembles that of the Sponges. 



In opposition to this I must, however, remark, that in this 

 case the immigration of amoeboid cells does not produce a 

 tissue such as we generally meet with in Sponges. No 

 fixed connective cells at all are formed. The immigrant 

 elements seem rather, from Kent's own showing, destined to 

 an increase by division or for spore-formation ; whilst in the 

 Sponges, besides the amoeboid wandering cells, which probably 

 serve for the formation of the sexual products, there occur 

 numerous other cells, which have attained special develop- 

 ment for different purposes, partly as connective corpuscles, 

 partly as contractile fibre-cells, partly as gland-cells , and partly 

 even as sense- and ganglion-cells (as lately stated by Von 

 Lendenfeld in the ' Zoologischer Anzeiger,' no. 186). 



As a histological difference of importance we have further 

 to note the circumstance that, as I first demonstrated, the 

 whole surface of the connective substance of the sponge-body, 

 which is bathed with water, so far as it is not occupied by 

 collared cells, is covered with a single layer of flat epithelial 

 cells, which either possess a smooth outer surface or bear each 

 a flagellura. Such a covering of flat cells is entirely deficient 

 in Protospongia. 



Finally, 1 will also call attention to the fact that in Proto- 

 spongia all the collared cells are immersed up to the collar in 

 the gelatinous uniting mass, while the corresponding cells of 

 the Sponges are only seated by their basal surfaces upon the 

 connective foundation, but otherwise stand freely side by 

 side. 



In turning now to the criticism of the agreement of the 



