THE ANIMAL-PLANTS. 137 



four perfectly different systems of organs ; by the intestines, 

 the vascular system, the organs of respiration, and tlie 

 urinary apparatus. In Zoophytes, however, these functions 

 and their organs are not yet separate, and are all performed 

 by a single system of alimentary canals, by the so-called 

 gastro-vascular system, or the coelenteric apparatus of the 

 intestinal cavity. The mouth, which is also the anus, leads 

 into a stomach, into which the other cavities of the body also 

 open. In Zoophytes the body-cavity, or "cceloma," possessed 

 by the four higher tribes of animals is still completely 

 M^anting, likewise the vascular system and blood, as also the 

 organs of respiration, etc. 



All Zoophytes live in water; most of them in the sea, only 

 a very few in fresh water, such as fresh-water sponges 

 (Spongilla) and some primaeval polyps (Hydra, Cordylo- 

 phora). A specimen of the pretty flower-like forms whicli 

 are met with in great variety among Zoophytes is given on 

 Plate VII. (Compare its explanation in the Appendix.) 



The tribe of animal-plants, or Zoophytes, is divided into 

 two distinct provinces, the Sponges, or SpongicB, and the Sea- 

 nettles, or Acalephce (p. 144). The latter are much richer 

 in forms and more highly organized than the former. In all 

 Sponges the entire body, as well as the individual organs, 

 are differentiated and perfected to a much less extent than 

 in Sea-nettles. All Sponges lack the characteristic nettle- 

 organs which all Sea-nettles possess. 



The common primary form of aU Zoophytes must be 

 looked for in the Protascus, an animal form long since 

 extinct, but whose existence is proved according to the 

 biogenetic principle by the Ascula. This Aseula is an 

 ontogenetical development form which, in Sponges as well 



