MATERIALS FOR A MONOGRAPH OV THK ASCONS. 493 



presence of vacuoles, containing bodies which, to judge frona 

 their appearance, consist of calcareous matter. The near 

 proximity to one of them (fig. 15) of what seems to be a much 

 corroded fragment of a spicule, has led me to the belief that 

 these porocytes may exercise the additional function of re- 

 moving, or rather absorbing, pieces of broken spicules. In 

 so far as they perform this office they might be termed 

 " scleroclasts.^' I make this suggestion with all caution, but 

 there seems to me no inherent improbability in the porocytes, 

 especially at an early stage in their differentiation, exerting a 

 scleroclastic function. 



A very important growing point in the sponge, as has been 

 said, is the oscular rim ; and here, too, porocytes are formed in 

 great numbers, and in a manner differing in details from that 

 which I have mentioned above as the typical method. The 

 oscular rim is, in fact, a region where the origin of porocytes is 

 so easy to study that it is a marvel to me that it should not have 

 been described long ago. For reasons which will be apparent 

 when we come to study the formation of the fourth ray of the 

 quadriradiates I propose to go into the matter rather fully, 

 and as a type I will take contort a, where the facts are very 

 plain. 



If a piece of the oscular rim of contort a be laid out flat 

 and examined from the dermal surface, flat epithelium of the 

 normal type is seen (PI. 41, fig. 40). If, on the other hand, the 

 oscular rim be examined from the gastral view, especially 

 rather near the limit of the collar-cells, an epithelium is seen 

 which at first sight appears very different (PI. 41, fig. 38). The 

 cells are packed with granules which obscure and often hide 

 the large pale nucleus, and the whole cell has a most pro- 

 nounced yellowish-brown tint. In fact, after what we have 

 already seen we can identify the cells without hesitation as 

 porocytes of a most typical kind. PI. 41, fig. 41, shows the two 

 types of epithelium in a decalcified section. On the lower 

 (outer) side we see the typical dermal epithelium, and on the 

 upper (inner) side we see the layer of granular porocvtic 

 epithelium, for so we may term it without further preamble, 



