682 PHYTOZOA. 



THIRD CLASS. PHYTOZOA, or ZOOPHYTA. (Gr. cpvwv, phuton, a 

 plant; ^woy, 20071.) PLANT-LIKE ANIMALS. 



The larger part of the animals composing this class are ma- 

 rine, some species are fixed to the soil. None of them are 

 properly free, swimming animals, although some of them can 

 move at will from their location. Portions of them are uncon- 

 nected with others, independent and single ; others are joined in 

 large societies, having the base of the stems in union. Some 

 have no hard-support ; others secrete a stony skeleton, termed 

 corallum, (coral,) thus constructing the well-known coral reefs 

 and islands, modifying "the shades of the ocean's depth, and 

 forming whole mountain ranges." The corals cannot properly 

 be regarded as the shells with which the animals cover them- 

 selves, the hard parts being, in reality, a part of the internal 

 structure. Prof. Dana, in his magnificent and standard work 

 on the Zoophytes, says "the corallum is entirely concealed 

 within the polyp, as completely as the skull of an animal beneath 

 its fleshy covering. All corals are more or less cellular, and 

 through the cellules, the animal tissues extend." In some in- 

 stances, however, the coral is exposed, i. e., when the increase 

 occurs from a terminal secretion upon a separate stem. As the 

 stem increases in length above, the part below dies. This in- 

 crease and disappearance of vitality above and below are com- 

 mon ; and thus are formed the huge masses of coral. The most 

 common species engaged in the production of coral banks, are 

 the Meandrina labyrinthica, (Brain Coral,) the Caryophyllia, 

 Madrepora, Forties, and Astrea, especially the latter. (See 

 Chart.) A solid dome of the Astrea, twelve feet in diameter, has, 

 according to Prof. Dana, a living exterior not more than a half or 

 three-fourths of an inch in thickness. "It is a well-known fact," 

 he says, (Silliman's Journal, Jan., 1837,) that corals cannot grow 

 above the surface of the water; and that reef-building corals 

 cannot grow at a greater depth than from ten to twelve fathoms, 

 or above the surface of the water at low tide ; therefore a coral 

 reef cannot be more than sixty or seventy feet thick." As a 

 condition of coral growth, "the sea-water must be pure and 

 transparent. Corals will not grow, therefore, on muddy shores, 

 or in water upon the bottom of which sediment is deposited." 



Some of the Polyps increase both by eggs and buds ; but not 

 all of the latter can in their turns produce eggs. They seize 

 their food with their tentacles; their whole surface is covered 

 with vibratory cilia so exceedingly minute as to be discernible 

 only through the aid of the microscope. These minute hairs are 



