ox MEJILIA XOEMANI. 667 



diameter, aud the smaller ones or pores about 20-30 fi. Both 

 kinds are oval or circular, and quite flush with the surface. 



The pores uiaj form a circle round an oscule (PI. 32, fig. 7), 

 but sometimes no regular arrangement is perceptible (fig. 8). 

 The regular plan of a central oscule and a rmg of pores would 

 seem to conform to the shape of the upper crypts and tubercles, 

 but the reverse is probably thf^ case, the crypts conforming to 

 the arrangement of the canal system, as I hope to make clear 

 later. 



Both oscules and pores are provided with a very well- 

 developed sphincter apparatus, consisting of concentric and 

 radial contractile cells for respectively closing and opening 

 the orifices. Some of these cells are remarkable in shape, 

 viz. with long processes which curve round the orifices and 

 which may actually anastomose, thereby becoming porocytes. 

 These cells seem to indicate the mode in which porocytes may 

 have arisen, viz. by the fusion of processes curving round an 

 orifice, and not by the appearance of a hole in the solid body 

 of the cell (PI. 37, fig. 3j. 



The incurrent canals pass down and bifurcate, passing right 

 and left (as seen in sections). Some of them present a 

 puzzling appearance, for they are surrounded by flagellated 

 chambers. Sometimes a string or tube of flagellated chambers 

 is seen traversing an open space. The lumen of the tube 

 is the incurrent canal and the open space is an excurrent 

 one, into which the apopyles on the outer surface of the tube 

 open. 



Fig. 13, on PI. 33, which depicts the growing edge of a very 

 young sponge, will show how this arrangement has probably 

 come about. In the youngest stage the canal structure is that 

 of a simple rhagon, i. e. of a gastral cavity with mucb folded 

 walls, the folds branching out to the periphery. The spaces 

 between and outside the folds constitute tlie incurrent, and 

 those in the lumen of the folds, channels or canals, the 

 excurrent system. 



The incurrent canals pass between the little clusters of 

 flagellated chambers, which clusters at this stage have a 



