STUDIES ON THE COMPAEATIVE ANATOMY OF SPONGES. 25 



action of weak hydrochloric acid. This view regarding the 

 nature of the spicule sheaths has been already expressed by 

 Haeckel and Schulze (11), and is doubtless the correct one. 



The mesodermal cells, which lie embedded in the ground- 

 substance, may be classified as follows : 



(1) Amoeboid. 



(2) Stellate. 



(3) Glandular: 



(a) Spicule-secreting. 



{b) Slime- or cuticle-secreting. 



(4) Endothelial. 



(5) Muscular. 



(6) Nervous. 



(7) Reproductive. 



Amoeboid Cells. — Concerning the amoeboid cells proper I 

 have no information to add to that which we already possess. 

 They are distinguished from the stellate cells by their more 

 rounded and massive form, and their more abundant and more 

 granular protoplasm. Certain of them, as is well known, 

 develop into ova, and these I shall describe later on. 



Stellate Cells. — These, which maybe regarded as the con- 

 nective-tissue cells of the sponge body, have also the usual 

 form, characterised by the long, slender, and often branched 

 processes given off from an inconspicuous central mass of 

 protoplasm surrounding the nucleus (fig. 26, st. c). It is very 

 probable that, as in the case of Stelospongos (2) and other 

 sponges, adjacent stellate cells may be united by their slender 

 processes, but I have not succeeded in clearly demonstrating 

 the connection here. 



Glandular Cells. — These are of two kinds — spicule-secret- 

 ing, and slime- or cuticle-secreting cells. We will consider the 

 spicule-secreting cells, or calcoblasts, as they have been termed 

 by PolejaefF (8), first. 



It is generally admitted that both calcareous and siliceous 

 spicules originate within special mother-cells, but probably in 

 both cases they subsequently receive additional layers of the 

 calcareous or siliceous material from other cells. Polejaeff (8) 



