120 



ZOOLOGY 



SECT. 



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

 deeply situated spaces (subcortical cavities), from which the incur- 

 rent canals lead to the ciliated chambers. This dermal cortex 

 present, though not highly developed, in Sycon gelatinosum 



is 



(Fig. 82, 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 



FIG. 89. Cells of the ectoderm (pinaco- 

 cytes) very higlily magnified. (After 

 Von Lendenfeld.) 



and very rarely assume other forms 



regarded as representing dermal 

 cavities. In most higher sponges 

 a special inner layer is developed ; 

 this is the gastral cortex, repre- 

 sented in a rudimentary form in 

 Sycon gelatinosum (Fig. 82, c/c) 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 (pinacocytes) of the 

 ectoderm (Fig. 89) are flattened, 



in some cases each flattened 

 flagellum. Lining the para- 



ectodermal cell is provided with a 

 gastric 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 

 mesoglcea are embedded con- 

 nective-tissue cells, amoeboid 

 wandering cells, and, in cer- 

 tain positions (around orifices), 

 muscle-cells. Unicellular glands 

 (see p. 26) are present in 

 some sponges, both calcareous 

 and siliceous ; also cells con- 

 taining the pigment to which 

 the bright colour of many 

 sponges is due, though in most 

 cases the pigment is not con- 

 fined to special cells, but occurs 

 scattered through the connec 

 tive-tissue cells and flagellate 

 cells. Fresh-water Sponges are often green, owing to the presence 

 of chlorophyll, the colouring matter to which the prevailing green 

 colour of plants is due. 

 The elements of the skeleton differ in character in the different 



IG. 90. Development of a tri radiate spicule of 

 Clathrina. scl, scleroblasts. (After Minchin.) 



