i5i 



CHAPTER X. 



SUB-KINGDOM II.—PORIFERA. 



Under the name of Porifera are included all those singular organ- 

 isms which are commonly known as Sponges. Originally regarded 

 as being of a vegetable nature, the Sponges are now universally 

 admitted to be animals ; though naturalists are not yet in absolute 

 agreement as to the precise position in the animal kingdom which 

 ought to be assigned to them. Owing to the close likeness of some 

 of the cell-elements of the Sponges to certain of the Protozoa, the 

 entire group has been often referred to this latter sub-kingdom. 

 Thus, some of the cells of a sponge are morphologically identical 

 with the Amcebce, while others present the closest possible resem- 

 blance to the Flagellated Infusoria. Hence, a sponge has often been 

 regarded as being a kind of colony, the units of which are morpho- 

 logically Protozoans. Naturalists are, however, now agreed as to the 

 removal of the Sponges from the Protozoa ; and they are by many 

 authorities regarded as forming the lowest division of the Zoo- 

 phytes (Coslenterata). Other authorities consider that the Sponges 

 represent a distinct morphological type, intermediate between the 

 Protozoa and the Cos/enterata, and that they are therefore entitled to 

 take rank as a separate sub-kingdom, to which the name of Porifera 

 has been given. In the present state of our knowledge, this view 

 seems to be the one which is attended with the fewest difficulties, 

 and it will therefore be followed here. 



The Sponges may be defined as multicellular organisms of variable 

 shape, the cells of ivhich are typically disposed to form an outer mem- 

 brane, an inner j?ie??ibrane, and an intermediate stratum ; a?id which 

 are traversed by canals, ivhich open o?t the surface, and which are 

 more or less extensively lined by flagellate cells. In most cases the cel- 

 lullar aggregate is supported by a framework of horny fibres, or of 

 flinty or calcareous spicules. A defi?iite motcth and stomach are want- 

 ing, and a nervous system is not known with certainty to be developed. 



The entire aggregate of cells which constitutes a sponge is so 



