112 ZOOLOGY SECT. 



of each of which (sp") frequently projects freely into the paragastric 

 cavity, covered over by a thin layer of flattened endoderm cells. 



The mesoglcea itself, as distinguished from the spicules which 

 lie embedded in it, consists of a clear gelatinous substance 

 containing numerous nucleated cells of several different kinds. 

 Most of these are small cells of stellate shape, with radiating 

 processes the connective-tissue cells or collencytes (Fig. 81, co) ; 

 others are fusiform ; a good many the amoeboid wandering cells 

 are Amoeba-like, and capable of moving about from one part of 

 the sponge to another. 



Around the inhalant pores and the apopyles are elongated cells 

 (Figs. 83 and 84), sometimes prolonged into narrow fibres. These 

 are contractile effecting the closure of the apertures in question 

 and are therefore to be looked upon as of the nature of muscular 

 fibres. In the case of the inhalant pores they are ectodermal ; in 

 that of the apopyles they are endodermal. A band of similar 

 fibres surrounds the osculum the oscular sphincter. 



The sexual reproductive cells the ova (Figs. 81 and 82, ov) and 

 sperms are developed immediately below the flagellate endoderm 

 cells of the flagellate canals, and in the same situation are to be 

 found developing embryos (em, em'), resembling in their various 

 stages those of Sycon raphanus, as described below. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



Sponges are plant-like, fixed, aquatic Metazoa, all, with the 

 exception of one family, inhabitants of the sea. The primary form 

 is that of a vase or cylinder, the sides of which are perforated by a 

 number of pores and in the interior of which is a single cavity ; 

 but in the majority of Sponges a process of branching and folding 

 leads to the formation of a structure of a much more complex 

 character. The surface of the Sponge is covered by a single layer 

 of flattened cells the ectoderm 1 and the internal cavities, or a 

 part of them, are lined by a second single layer the endoderm 

 part or the whole of which consists of a single layer of choanocytes, 

 i.e. columnar collared cells, each provided internally with a long 

 flagellum. Between these two layers is a quantity of tissue 

 usually of a gelatinous consistency the mesoglcea containing a 

 number of cells of various kinds. The wall of the Sponge is 

 pierced by a number of apertures. The skeleton or supporting 

 framework, developed in the mesoglcea from cells derived from 

 the ectoderm, consists in some cases of fine, flexible fibres of a 

 material termed spongin ; in others of spongin-fibres supplemented 

 by microscopic siliceous spicules ; in others of siliceous spicules 

 alone ; in others of spicules of carbonate of lime. Keproduction 

 takes place both asexually by the formation of gemmules, and 



1 See footnote on p. 109. 



