SILICIOUS AND HORNY SPONGES WILSON. 



289 



metrically toward the center of the sponge (pi. 37, fig. 6, right). But 

 in some specimens it extends inward very obliquely (pi. 37, fig. 6. 

 left). 



In a small yet considerable minority of the specimens a distinct 

 cloaca is not developed. Instead, the osculum, remaining apical as 

 usual, is simply the aperture of a single small efferent canal. Or in 

 place of cloaca and osculum there is only a most minute depression 

 into which several small efferent canals open. Such specimens occur 

 along with the common type in the same locality. Many of them 

 are small, but some are of good size, 10-14 mm. diameter. There is 

 intergradation between the two classes in the same locality. The 

 facts indicate that with continued growth there is a strong tendency 

 in the species to* develop a cloaca, but that this tendency is inhibited, 

 or possibly is germinally weak, in some individuals, which therefore 

 reach a considerable size, 14 mm. in diameter or thereabouts, without 

 developing a cloaca. 



Table giving data for nine individuals concerning variation in the cloaca. 



The spicules, in respect to details of shape and dimensions, are 

 not far from those recorded by Ridley, Sollas, etc., as may be seen 

 from the following data. 



