48 SPONGES 



for where contractile cells are differentiated, the existence also of special 

 nerve cells is at least possible. It can only be said that the existing 

 statements with regard to sense cells in sponges are, for the most part, 

 quite untrustworthy, both in matters of fact and observation, as well as 

 of interpretation, and that a complete scepticism with regard to this 

 point is not only justifiable, but even demanded, in the present state of 

 the question. 



(2) Porocytes. The description given above for the pore cells 

 of the Olyntlius may be extended, in its main features, to those of 

 all Calcarea. The porocy tes are large, coarsely granular, cells, very 

 contractile, and capable of considerable migration by amoeboid 

 movement. The pore duct arises by an intracellular perforation. 

 In Heterocoela the porocytes form the chamber pores or prosopyles, 

 the so-called dermal pores being intercellular ostia. 



In Clathrinidae the pores are situated on the surface of the body, on 

 a level with the dermal epithelium, but in Leucosoleniidae and in the 

 allied Heterocoela (Sycon, Leucandra, etc.) the pore is placed at the inner 

 end of a funnel-shaped depression, forming a short afferent canal. For 

 such cells Bidder (1) has suggested the term pylocytes. 



The origin of the porocytes, from the dermal epithelium, and 

 especially from that lining the oscular rim, has been described 

 above. In the latter region the same cell layer which furnishes 

 porocytes gives rise also to sphincters or contractile membranes 

 for closing the oscular opening, a fact \vhich emphasises the con- 

 tractile nature of the pore-forming cells. Besides their contractility, 

 a remarkable feature of the porocytes is the readiness with which 

 they give rise to skeletal structures of various kinds. Thus in 

 Clathrinidae (and all Calcarea 1} the fourth or gastral rays of the 

 quadriradiate spicules are secreted each by a porocyte (see below, p. 

 108, Fig. 75, 4 and 5). Moreover, in many Calcarea (e.g. Clathrina 

 coriacea, encrusting form) the porocytes pass into the gastral cavity 

 between the collar cells, and give rise to a cellular network ramify- 

 ing throughout the whole gastral lumen. The strands of the net- 

 work are composed of porocytes placed end to end, and the axis 

 of each strand contains a fibre which has the same staining 

 reactions as the sheaths and axial threads of the spicules. The 

 fibres appear to be formed as an intracellular secretion of the poro- 

 cytes, which in this way furnish an elastic framework for the 

 support of the delicate sponge body. 



The porocytes were long overlooked or interpreted erroneously in 

 Calcarea, and great doubt still attaches to their existence in non- 

 calcareous sponges. A comparison with Calcarea would guide us to seek 

 for them in the prosopyles, but there is as yet no proof that the proso- 

 pyles in siliceous sponges are intracellular ducts. Most authors have 

 been unable to decide definitely as to the nature of the prosopyles, but 



