THE ZOOPHYTES. 481 



developed, and the gills exposed. The organs consist in a 

 crown of tentacles, which surround the mouth, and which have 

 laterally vibratile cilia (Fig. 464). The anus is near the 

 mantle, and the blood arrives between the viscera and the 

 mouth, as well as in the interior of the tentacles, but is not 

 set in motion by a heart. Finally, the inferior portion of the 

 mantle is generally hardened, so as to form a tube or cellule, 

 sometimes homy, sometimes calcareous, into which the animals 

 may retire altogether. In general these beings, so small as 

 to be almost microscopic, live reunited in masses more or less 

 considerable. Most of them dwell in the sea, but some live 

 in fresh waters. Amongst these last we may mention the 

 alcyonellae, the plumatella (Fig. 464), common enough in our 

 stagnant waters; and amongst the first, the flustra, the 

 retepora and the vesicularia. 



PRIMARY 

 THE ZOOPHYTES. 



616. In this, the fourth and last primary division of the 

 animal kingdom, the organization is much less complete than 



Fig. 4.66. Oursin (the Echinus, or Sea Hedgehog).* 



* On the left side the spines have been removed to show the shell. 

 I I 



