Ill PHYLUM AND CLASS PORIFERA 119 



is increased by the development of a variety of infoldings of the 

 ectoderm which appear in the higher forms. The oscula dis- 

 tributed over the surface of the mass may indicate the component 

 zooids,but these are not always recognisable, being carried inwards 

 , by the infoldings or closed up altogether. 



A thicker or thinner specialised outer layer — the dermal cortex 

 — situated immediately below the superficial ectoderm, is present 

 in many Sponges. This is a layer of, mesogloea with special 

 skeletal elements, usually containing spaces and canals lined by 

 ectoderm — {suMcrmed cavities, Fig. 87, SB) — which communicate 

 directly with the exterior, and, internally, usually with more 

 deeply situated spaces {subcortical cavities), from which the in- 

 current canals lead to the ciliated chambers. This dermal cortex 

 is present, though not highly developed, in Sycon gelatinosum 

 (Fig. 81, dc), and the enlarged outer ends of the incurrent canals 

 lying in the dermal cortex and closed externally by the pore- 

 bearing membrane, may be regarded as representing dermal 

 cavities. In most higher sponges a special inner layer is 

 developed ; this is the gastral cortex, represented in a rudi- 

 mentary form in Sycon gelatinosum (Fig. 81, gc.) as the internal 

 layer with special spicules, in which the excurrent canals are 

 situated. 



Histology. — In the protoplasmic elements or cells of the 

 various groups of Sponges there is little variation, except 

 in minor points. The cells 

 of the ectoderm (Fig. 88) 

 are flattened, and very rarely 

 assume other forms; in some 

 cases each flattened ecto- 

 dermal cell is provided with a 

 flagellum. Lining the paraga- 

 stric cavities and canals is a 

 layer of flattened cells similar 

 to those of the ectoderm, or of 

 flagellate collared cells. In 

 the gelatinous substance of the 

 ,-j- mesoglcea are embedded connec- j.^^ ss.-cdis of the ectoderm, very highly 



,y tive-tisSUe cells, amoeboid wan- magnified. (After Von Londenfold.) 



dering cells, and, in certain 



positions (around orifices), muscle;;cells. Unicellular glands (see 

 p. 25) are present in some sponges, both calcareous and siliceous ; 

 also cells containing the pigment to which the bright colour 

 of many sponges is due, though in most cases the pigment is not 

 confined to special cells, but occurs scattered through the con- 

 nective-tissue cells and flagellate cells. Fresh-water Sponges are 

 green, owing to the presence of chlorophyll, the colouring matter 

 to which the prevailing green colour of plants is due. 



