Animal Evolution. 191 



polarity. A more complex phase of arborescent 

 animal life is manifested by the polyzoa, zoophytes, 

 and sponges. Of these the most remarkable, if not 

 the most enigmatical of all living things, is the 

 polyzoon christabella. This is a sea-mat, somewhat 

 like a square-shaped snail, on whose back the 

 clustered polypides project their horse-shoe shaped 

 crown of tentacles ; while the base is a muscular foot 

 by whose aid the whole colony crawls along. It is 

 difficult to conceive how, without calling in the aid of 

 automatism, volition could be communicated to its foot 

 by an animal possessing such a multitudinous head. 



The horny zoophyte, nextly, buds like the infusoria 

 and fresh-water hydras, but instead of breaking off 

 and starting an independent existence on attaining 

 maturity it remains attached ; hence a huge plant- 

 animal eventually grows, each bud duly provided 

 with a separate mouth, but only enjoying the privilege 

 of a stomach in common. 



Sponges also form colonies and secrete a horny 

 supportive net-work, while their collective action is 

 strikingly mechanical : consisting in sucking water 

 through innumerable small holes in the external 

 surface, and after extracting all floating nutriment, 

 pumping it out again through larger orifices. 



In seeking a cause for arboriform growth among 

 animals, we find that the carbonate of lime in the 

 animal skeleton is not entirely acting on its own 

 account. In many instances, if not in all, the action 

 is but the hardening of the creature's internal tissues 



