120 



ZOOLOGY 



SECT. 



FIG. 89. Cells of the ectoderm (pinaco- 



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 

 is present, though not highly developed, in Sycon gelatinosum 

 (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 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, 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. 



*- *-"J , V-"--i*0 WA UiiVj %y\jl<l_/xav,l 111 \llLHtl\j\J- rrm -| -. , . 



cytes) very highly magnified. (After 1 h 6 Cells (pindCOCllteS) O I the 

 Von LendenfeJd.) j ,-TK on\ n T 



ectoderm (Fig. 89) are flattened, 

 and very rarely assume other forms ; in some cases each flattened 

 ectodermal cell is provided with a flagellum. Lining the para- 

 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 



I'm. 0',). Development of a tri radiate sijicule (if 

 Clathrina. sd, scleroblasts. (After .Minchin.) 



