36 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Leucon, Sycon. The commonest British form is 

 ordinarily known as Grantia. (See Fig. 7.) 



3. Silicispongise, in which part of the skeleton 

 is made up of spicules of silica ; e.g. the common 

 fresh-water sponge (Spongilla), Chalina, Euplectella. 



4. Ceratospoiigiae, in which the skeleton is 



Fig. 8. A, Hydra v : ridis, attached to Duckweed; B, a Single Specimen 

 magnified ; c, Hydra in Diagramatic Section. 



cc, Ectoderm ; en, endoderiu ; m, mouth ; be, enteric cavity ; t, tentacles. 



completely horny or fibrous, and devoid of siliceous or 

 calcareous spicules ; e.g. the bath-sponges (Euspongia). 



In the Coelenterata it is otherwise; in many 

 forms both nervous and muscular tissues are to be 

 recognised not only by the aid of the microscope, 

 but by the activity of these animals, and by their 

 reactions when subjected to physiological experiment. 



Henceforward, then, we have to do with forms 

 which possess, in some shape or other, all the 

 essential tissues of even the most complicated 



