110 



ZOOLOGY 



SECT. 



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 mesoderm with special 

 skeletal elements, usually containing spaces and canals lined by 

 ectoderm suMcrmal cavities (Fig. 78, 8D) which communicate 

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

 situated spaces (stibcortical cavities), from which the incurrent canals 

 lead to the ciliated chambers. This dermal cortex is present, 

 though not highly developed, in Sycon gelatinosum (Fig. 72, dc), 

 and the enlarged outer ends of the incurrent canals lying in the 

 dermal cortex, and closed externally by the pore-bearing mem- 

 brane, 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 rudimentary form in Sycon gelatino- 

 sum (Fig. 72, 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. 

 79) are flattened and form a 

 syncytimn ; very rarely they as- 

 sume other forms ; in some cases 

 each flattened ectodermal cell is 

 provided with a flagellum. The 

 endoderm consists of flattened 

 cells similar to those of the ecto- 

 derm, or of flagellate collared 

 cells. In the gelatinous sub- 

 stance of the mesoderm are em- 

 bedded connective-tissue cells, 

 amoeboid wandering cells, and, 

 in certain positions (around 

 orifices), muscle cells. Uni- 

 cellular glands (see p. 22) are present in some sponges, both cal- 

 careous 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 connective-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. Sensory cells or nerve cells have been described ; but the 

 nature of the elements which have been so regarded cannot be 

 said to have been placed beyond question. 



FIG. 79. Cells of the ectoderm, very highly 

 magnified. (After Von Lendenfeld.) 



