STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 223 



the outside. I do not doubt, however, that there is always a 

 thin layer of mesoderm between the two, a fact which is 

 indicated by the development of spicules in the chamber walls. 

 It is certainly continued into the endogastric septa of 

 Grantessa erinaceus, which consist of a layer of gelatinous 

 ground substance, with an occasional stellate cell, covered 

 on each side by a layer of flattened epithelium (fig. 62). 



Concerning the spicule sheaths, which are formed by a 

 slight concentration of the structureless mesodermal jelly 

 around the spicules (fig. 53), I have nothing to add. 



In my memoir on Grantia labyrinthica I classified the 

 mesodermal cells which lie embedded in the gelatinous ground 

 substance as follows: — (1) Amoeboid, (2) Stellate, (3) Glan- 

 dular, (4) Endothelial, (5) Muscular, (6) Nervous, and (7) Re- 

 productive. I am still prepared to abide by this classification 

 so far as the amoeboid, stellate, glandular, and reproductive 

 cells are concerned. The only muscular cells which have yet 

 been observed in calcareous sponges I now regard, as already 

 indicated, as ectodermal or endodermal. The supposed 

 nervous cells I am now extremely doubtful about, in all cases, 

 and I am strongly inclined to think that the appearances 

 described as such are due to the presence of subdermal gland- 

 cells. This point, however, can only be settled by a very 

 careful re-investigation of living material. The endothelial 

 cells I still retain provisionally amongst the mesodermal 

 elements, although I shall presently give reasons for believing 

 that they need not necessarily be regarded as such. 



Amoeboid and Stellate Cells. — I have observed most 

 beautiful examples of amoeboid cells inLeucandraechinata 

 (fig. 33) and L. phillipensis (figs. 48 — 50). They are found 

 scattered here and there in the mesodermal ground substance 

 between the flagellated chambers, and, in these two sponges, 

 are conspicuous by their enormous size. The amoeboid cells are 

 typically distinguished from the stellate cells by their more uni- 

 formly granular protoplasm, their larger nuclei, and the absence 

 of the slender, thread-like, radiating processes, which charac- 

 terise the stellate cells ; the latter are very difi'erent in appear- 



VOL. 85, PART 2. — NEW SEE. R 



