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112 



THE SPONGES. 



In each of the dermal areas embraced by the cladomes of the dicho- 

 triaenes there is usually a single pore (Fig. 11, Plate 15). A few areas are 

 without pores, and here and there a large area containing two or three pores 

 occurs. The pores open into radial pore or ectosomal canals. These are 

 uniporal, and open without chone-like constrictions into subdermal chambers 

 (Fig. 9, Plate 15), which again open without chone-like constrictions into 

 narrow canals passing into the interior. The subdermal chambers are 

 numerous, subspheroidal, connect with one, or sometimes two pore canals, 

 and lie in the zone where the ectosome passes into the choanosome. 



There is no highly specialized cortical fibrous layer, but there is an 

 ectosome devoid of flagellated chambers, and about 200-500 /x, thick. 

 It consists of collenchyma, containing very numerous cells, which have a 

 granular body and slender processes. In sections radial to the surface of 

 the sponge, the cells appear spindle-shaped ; the processes long, delicate, 

 and fibre-like, frequently extending parallel to the surface. Such tan- 

 gentially extending fibre-cells are sufficiently abundant throughout the 

 ectosome to imprint a fibrous character upon it. In the immediate neigh- 

 borhood of the subdermal spaces the fibre-cells are pretty thickly packed, 

 and extend tangentially to the wall of the space. 



The ectosome exhibits, in radial sections, small canals here and there, 

 some of which are cut lengthwise, others obliquely, or transversely. 

 " Blaschen," such as are present in Penares (Ecionema) hetteri (Lendenfeld, 

 1894, p. 39), are not present. The ground substance of the ectosomal 

 collenchyma consists chiefly of a homogeneous material, which includes 

 some of the fine granules so abundant in the choanosome. The granules 

 often show an arrangement in vaguely marked groups. 



The mesenchyme of the choanosome is dense, although everywhere 

 excavated by small canals and flagellated chambers. The ground substance 

 consists of a homogeneous matrix strewn with fine granules. The gran- 

 ules are angular, less than 1 \l in diameter, and show with an immersion a 

 highly refractive dot in the centre. They are in general very abundant ; 

 but are by no means evenly distributed, and tracts of comparatively clear 

 matrix occur commonly. Cells with pseudopodia, and occasionally rounded 

 egg cells (Fig. 5, Plate 15) with large nucleus are found imbedded in the 

 ground substance. The egg cells are surrounded by a fibrous layer, 

 outside of which is a layer of ground substance, which contains but few 

 granules. A similar clear layer lies round all cells. 



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