266 THE MEDALS OF CREATION. Cuap. VII. 
nor capable of contraction and expansion, as in the Anthozoa: — 
it consists of a digestive cavity or sac, which is bent on itself 
and open at both extremities. The outer integument is 
either membranaceous or horny ; ; Sometimes cal aeee The 
oral aperture or mouth is surrounded by a circle of tenta-_ 
cula, from eight to twelve or more in number, and these : 
tentacles are ‘othed with vibratile cilia. (Wond. p. 606, — 
the polype of Plustra pilosa.) 
The polypes in this order never occur singly ; they are | 
always united by a common integument, but each enjoys an 
individual existence. The animal can extend its tentacula 
and protrude the mouth from the cell, but the rest of the 
body is incapable of extension or contraction. These poly-— 
pifera increase by germination. In their organization, they 
so closely approach the mollusca, that in recent zoological 
systems they are placed in that class. The ciliated cha- 
racter of their tentacula has also led to their being named — 
Cilio-branchiata. But as it is desirable in a work of this” 
elementary nature to avoid conflicting opinions as much as” 
possible, the fossil Bryozoa will be considered as corals, in — 
the general sense of that term. 
Frustra (Sea-mat). Lign. 89, fig. 4,5.—The polyparium — 
is either membranaceous and flexible, calcareous and en-_ 
crusting, or foliaceous, composed of cells, arranged in juxta-_ 
position, more or less quadrangular, flat, with a distinct 
border, disposed on a flat surface, or on opposite surfaces, as_ 
in the F. foliacea. ‘ 
This is one of the most common genera of the encrastiiill 
and frondescent zoophytes. The /Justra consists of a cluster, 
or aggregation of polypes, invisible to the naked eye ; under 
the microscope, the polype is found to bea transparent gela- 
tinous body bent on itself, with a sac or digestive cavity, 
having two apertures, the external margin of which termi- 
nates in eight or ten tentacula, clothed with cilia; the 
whole is surrounded by a firm wall, constituting a cell, from _ 
7 
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