ACANTHASCUS CACTUS. 145 



of the sponge to a cactus is so remarkable that one is at once 

 impressed with the appropriateness of F. E. Schulze's selection 

 of the specific name. No doubt all the cones arise in connection 

 with the prostalia, but it is by no means uncommon that we meet 

 with such as show no trace of the latter, which evidently have 

 been lost. Cones without prostalia are especially frequent in the 

 larger and therefore older individuals ; in fact in these it is rare 

 that we find any of the prostal needles at all. The loss is 

 undoubtedly partly due to a mechanical breaking off at the apex 

 of the cones due to some external cause, as is proven by the 

 proximal part of the needles still remaining in the cones, the 

 outer end being broken off. At the same time I am inclined to 

 assume that a large part of the loss is to be accounted for by the 

 fact that the spicules in their entirety are cast off by some 

 natural physiological process of the sponge. Only in this way 

 can be explained the entire absence of even the proximal remnant 

 within the cones in so manv cases. Probablv the lono-er and 

 therefore older prostals are the first to be thrown out, and in 

 sujDport of this theory stands the fact that those still remaining 

 on the larger specimens are not so long nor so strong as some 

 to be seen on the smaller individuals. If I am right in the above 

 assumptions, it follows that the period of the greatest develop- 

 ment of the prostalia is when the sponge is comparatively small 

 and young, or at any rate before its full size is attained — a fact 

 which is not without parallel in other Acanthascinae {S. glaber, R. 

 victor, li. capillatus). 



Near the oscular margin there may occur some fine prostalia 

 which project either singly or in small tufts without having cones 

 at bases. They are however never so numerous as to form anything 

 like a fringe to the osculum. 



