PROGRESS OF MICROSCOPICAL SCIENCE. 239 



from the sponge, distinguisliable from a common amoeban animal. 

 Again, as regards the general form of the sponge itself, there are 

 many instances where the same form may be assumed by totally 

 different species, and the same species assume different forms, so that 

 a microscopical examination of the " proper spicule " can alone 

 determine the species ; thus a fan-shaped and a vase-like form re- 

 spectively may have at one time the same, and at another a different 

 form of spicule. And yet again the aid derived from the form of the 

 " proper spicule " is confined to sponges so provided, while those 

 which have nothing but foreign objects instead of the "proper 

 spicule" are even without this aid. So that, after all, we may be 

 thrown back upon structural peculiarities in combination with 

 general form, and perhaps sometimes colour, for ultimate distinction. 

 (This will be found to be particularly the case with the Hircinida.) 



Still there are many instances where the same species may be 

 hastily recognized by its outward features ; but as this can only be 

 done after much experience, it is of no use to a beginner. At the 

 same time, from what has been above stated, it would always remain 

 uncertain, even to the experienced, without a microscopical examin- 

 ation. 



A fresh sponge, too, described in its natural state (that is, with the 

 sarcode on), differs greatly from that in which the sarcode is off, or 

 where the skeleton only remains. As, however, by far the greater 

 number of sponges come to us in the latter state, and, indeed, all must 

 be divested of the sarcode before they can be usefully described for 

 classification, seeing that, as before stated, there is no animal ex- 

 pression (so to term it) externally or internally that can be made use 

 of for this purpose, it seems best to describe the skeleton naked, 

 rather than under cover of the sarcode — that is, to describe the 

 skeleton only, although, of course, where this can be done with the 

 sarcode on as well as off it is best of all. But there is no doubt that 

 a description of the sponge with the sarcode on will never serve to 

 recognize its skeleton, which is at once the most characteristic and 

 frequently the only part that we can or are ever likely to obtain from 

 the inaccessible localities in which many grow; so after all we. are 

 not so badly off with the skeleton only, provided it lias not been worn 

 away by much attrition. Hence the fundamental divisions of my 

 arrangement will be based on the characteristic features presented by 

 the elementary composition of the skeleton or organ of sujjj^ort. It 

 should not be forgotten, however, that with the sarcode of course the 

 flesh-spicules disappear, falling through the skeleton, as before stated, 

 like small pebbles through the meshes of a fishing net, when the 

 sarcode passes into dissolution. Nor should it be forgotten that 

 there may be a great difference between a sponge in its "fresh" and 

 in its dried state, in size, colour, and general appearance. As the 

 sarcode in all assumes the character of glue when dry, those which, 

 like the Carnosa, are without horny skeletons can only be described 

 when fresh or preserved in some aqueous solution. Also sponges 

 possessing a skeleton sink down in many instances to half their 

 original size by the shrinking up of the sarcode, which, clinging 



VOL. XV. S 



