THE ZOOPHYTES. 525 



and aggregated. These last have often a phytoid appear- 

 ance. 



The history of the biphores presents a very remarkable 

 peculiarity. Successive generations do not resemble each 

 other, but are composed alternately of aggregated and solitary 

 individuals. The first are hermaphrodite, and produce each 

 a younger one, which lives isolated, but which has no repro- 

 ductive organs, and gives birth, by granulation, to a sort of 

 chain of aggregated individuals. These singular animals are 

 sufficiently common in the Mediterranean. 



615. The bryozoaria, which even very lately have been 

 confounded with the more simple polypi, have the mantle less 

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

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

 laterally vibratile cilia (Fig. 550). 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 horny, 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 

 alcyonellse, the plumatella (Fig. 550), common enough in our 

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

 retepora and the vesicularia. 



PKIMAKY DIVISION. 

 THE ZOOPHYTES. 



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

 animal kingdom, the organization is much less complete than 

 in most other animals ; and the different parts of the economy, 

 instead of being disposed in pairs on each side of a longitudinal 

 plane, are grouped around an axis or central point, so as to 

 give to the whole of the body a radiated or spherical form. 

 The nervous system is either rudimentary or wanting ; and 

 there are no special organs of sense, unless it be certain small 

 coloured spots, bearing some analogy to the eyes of the 



