102 ZOOLOGY SECT. 



oxeote spicules lie embedded, is termed the dermal cortex (dc). A 

 thick stratum at the inner ends of the canals and immediately 

 surrounding the paragastric cavity is termed the (/astral cortex (gc). 

 It is supported by triradiate and also by tetraradiate spicules, one 

 ray of each of which (sp") frequently projects freely into the para- 

 gastric cavity, covered over by a thin layer of flattened endoderm 

 cells. 



The mesoderm itself, as distinguished from the spicules which 

 lie embedded in it, consists of a clear gelatinous substance con- 

 taining 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. 71, co); others are fusi- 

 form ; 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. 73 and 74), 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 endodennal. A band of similar 

 fibres surrounds the osculum the oscular sphincter. 



The sexual reproductive cells the ova (Figs. 71 and 72, 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 raplianus, 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 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 columnar 

 cells each provided internally with a long flagellum. Between these 

 two layers is a quantity of tissue usually of a gelatinous consistency 

 the rncxixli / ///, or mesoglcea containing a number of cells of 

 various kinds, some of which secrete the elements of the skeleton. 

 The skeleton or supporting framework, developed in the mesoderm, 

 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 



