Species of Echinonematous Sponge. 



51 



that the forms so revealed were always similar to that of the 

 spicule from which they were derived, a conical acuate always 

 remaining a conical acuate, and a stunted cylindrical one 

 always remaining stunted and cylindrical ; and thus Bower- 

 bank's notion that the latter are young forms of the former, 

 and would become pointed and conical with growth, is refuted; 

 the cylindric are aborted, not immature forms of the acuates. 

 The statement that residual acuates are of the same or similar 

 form to the original ones may appear difficult to verify, since 

 when the outer coat has been removed there is nothing 

 left to show what the form originally was ; and since the solu- 

 tion takes place while the spicules are being boiled, one can- 

 not witness the removal of a coating. That is true : but it is 

 a remarkable fact that solution does not wholly remove the 

 outer coat ; in other words, the solution is partial only ; for 

 after the potash has dissolved away the silica of one of the 

 envelopes there remains behind a less soluble residue, which 

 has the appearance of a delicate, soft, membranous film, and 

 Avhich retains very closely the original form of the envelope 

 from which it was derived. Thus in fig. 2 we have a sketch 



Fio-. 2. 



Smooth acuate after treatment with potash (x 140). 

 spicule ; f, residual membrane. 



s, residual 



of the residual film of an acuate enclosing within its slightly 

 folded, delicate, almost invisible substance (f) the solid, 

 strongly defined spicule (s). In fig. 3 an axial spicule is re- 

 presented with the residual film of an outer envelope washed 



Axial spicule after boiling with potash, s, remainder of spicule ; 

 /, outer sheath separated from it ; a, air-bubble (x 435). 



from it and lying on one side; the air-bubble (a) within this 

 sheath indicates the tenuity of its walls. 



What, then, is the nature of this residual substance, which. 



4* 



