12 ON CLASSIFICATION. 



is a most remarkable circumstance that though these animals are 

 abundant, and are constantly under observation, we are still in 

 doubt upon that essential point, — still uncertain whether there 

 may not be some phase in the cycle of vital phenomena of the 

 Ehizojooda with which we are unacquainted; and, under these 

 circumstances, a perfect definition of the class cannot even be 

 attempted. 



The next division is the group of the Spongida, which exist 

 under such multitudinous forms in both salt and fresh waters. 

 Up to the last few years Ave were in the same case, with respect 

 to this class, as with the Gresarinida and the Rhizopoda. Some 

 zoologists even have been anxious to relegate the sponges to the 

 vegetable kingdom ; but the botanists, who understood their 

 business, refused to have anything to do with the intruders. 

 And the botanists were quite right ; for the discoveries of late 

 years have not left the slightest doubt that the sponges are 

 animal organisms, and animal organisms, too, of a very con- 

 siderable amount of complexity, if we may regard as complex 

 a structure which results from the building up and massing 

 together of a number of similar parts. 



The great majority of the sponges form a skeleton, which is 

 composed of fibres of a horny texture, strengthened by needles, 

 or spicula, of silicious, or of calcareous, matter ; and this frame- 

 work is so connected together as to form a kind of fibrous 

 skeleton. This, however, is not the essential part of the animal, 

 which is to be sought in that gelatinous substance, which invests 

 the fibres of the skeleton during life, and is traversed by canals 

 which open upon the surface of the sponge, directly or indirectly, 

 by many minute, and fewer large, apertures. 



If I may reduce a sponge to its simplest expression — taking 

 the common Spongilla, for example, of our fresh waters, — the 

 structure — removing all complexities, and not troubling our- 

 selves with the skeleton, because that has nothing to do with 

 what we are now considering — may be represented by the 

 diagram (A, Fig. 3). There is a thin superficial laver (a) 

 formed entirely of a number of the so-called sponge particles, or 

 ultimate components of the living substance of the sponge each 

 of which is similar to an Amoeba, and contains a nucleus. These 



