THE WONDERS OF LIFE 



other structures co-operating in the rest of the coelenteria. 

 The characteristic stem of the sponges is distinguished 

 by the piercing of the wall of the gastric vesicle with 

 several holes. Through these water pours into the body, 

 bringing with it the small particles of food which are 

 received and digested by the ciliated cells of the entoderm ; 

 the water emerges again by the mouth-opening {osculmn). 

 The best-known of the sponges is the common bath- 

 sponge (euspongia officinalis), the horny skeleton of 

 which we use daily in washing. In these and most other 

 sponges the large, unshapely body is traversed by a 

 number of branching canals, on which there are thou- 

 sands of tiny vesicles, produced by the multiplication 

 of a simple gastric vesicle of the primitive sponge 

 (olynthus). Each of these ciliated chambers is really a 

 tiny gastraea, a "person" of the simplest character {cf. 

 chapter vii.)- Hence we may regard the whole sponge- 

 body as a gastraead-stock {cormus). 



The large group of the cnidaria offers a long series of 

 evolutionary stages, from very small and simple to very 

 large and elaborate forms. Some of them remain at a 

 very low stage, as does our common green fresh-water 

 polyp {hydra viridis), which only differs from the gastraea 

 by a few variations in tissue and the formation of a 

 crown of feelers about the mouth. Most of the polyps 

 form stocks (cormi), the individuals shooting out buds 

 which remain joined to the mother animal. In these 

 and all the other stock-forming animals the nutrition 

 is communistic ; all the food that the individuals get and 

 digest is conducted by tubes to the common fund and 

 equally distributed. In all the larger cnidaria the body- 

 wall becomes thicker, and is traversed by branching 

 gastro-canals ; these convey the nutritive fluid to all 

 parts of the body. 



While the fundamental type in the cnidaria is radial 

 (determined by the crown of radiating feelers or tentacles 



224 



