430 The Structure and Life-History of a Sponge. [Sess. 



term of zoophytes, or " animal-plants," was invented. Follow- 

 ing on the careful scientific methods of modern times, sponges 

 have now been conclusively relegated to the animal kingdom, 

 though a few obscure forms of life among the Infusoria, to some 

 of which sponges are closely allied in their minute structure, 

 remain as yet very much of a puzzle to zoologists. Still, modern 

 scientific research has supplied us with indubitable evidence of 

 the true nature and affinities of this large family of the sponges. 

 The elements of the sarcode or sponge-flesh, for example, with 

 which the fibrous or mineral skeleton is invested and filled up 

 during life, and which are also the active agents in building up 

 these horny and solid parts, are found to be identical morpho- 

 logically with the fresh-water amceba, or the gelatinous con- 

 tents of the Polycistina, the Foraminifera, and allied forms. 

 The further fact, that for the first time in the animal kingdom 

 true differentiated tissues are here met with, has removed the 

 sponges from the simple unicellular Protozoa to the higher 

 Metazoa or multicellular animals. But here a new difficulty 

 has arisen, for some are still inclined to include sponges among 

 the Ccelenterata, or " Zoophyte " group, in the modern scientific 

 meaning of that term. Most naturalists, however, have agreed 

 to place them, provisionally, in a separate class, between the 

 Protozoa and the Ccelenterata, and at the bottom of the Metazoan 

 sub-kingdom, under the class-name of Porifera or pore-bearers. 

 Having thus glanced at the true affinities of the sponges, 

 and the position which they occupy in the scheme of nature, 

 we may now go on to consider briefly their structure. 

 Workers with the microscope among " pond-life " are familiar 

 with a curious tiny creature, commonly present in stagnant 

 water, which seems to be little more than a speck of ani- 

 mated jelly. The various shapes which this creature assumes 

 as it glides over the field of the microscope are very amusing, 

 here throwing out a foot or a hand and there elaborating a 

 mouth from any part of its surface indifferently. This 

 Amceba, or Proteus Animalcule, as it is termed, is structurally 

 identical, as already said, with the fundamental tissue or vital 

 element of the sponges, by whose aid the whole superstructure 

 is built up. It is remarkable, indeed, that in nature beautiful 

 structures are so often reared by apparently insignificant 

 causes, as exemplified in the Diatomacere, where the contained 



i 



