184 INVERTEBRATE ANIMALS. 
Chae, ER OX, 
MOLLUSCOIDA. 
CLASS I. POLYZOA.—The members of this class are the lowest 
of all the Wodlusca, and they are generally known by the popu- 
lar names of “‘Sea-mosses” and “ Sea-mats.” They are invari- 
ably compound, forming associated growths or colonies, each 
consisting of a number of distinct but similar zodids, produced 
by gemmation from a single primordial individual. The colo- 
nies thus produced are very generally protected by a horny or 
chitinous integument, and they are so like the Hydroid Zoophytes 
that they were long described as such. The only absolute dis- 
tinction between the two classes is to be found in the internal 
structure of the zodids of each; but they may be generally 
separated by the fact that the separate cells in a compound 
Hydroid are all united to one another by means of a common 
flesh or ccenosarc ; whereas in the Polyzoa the separate cells 
composing the colony are merely connected externally, but 
very rarely have any direct communication with each other. 
The separate beings or zodids which collectively constitute the 
colony of any Polyzodn are spoken of as “polypides ”—the 
term folypite being only used in connection with the Hydrozoa, 
and the term folyfe being similarly restricted to the Actinozoa. 
Each polypide in a typical Polyzodz has the following struc- 
ture (fig. 95, 2): The body of the animal is enclosed in a 
double-walled sac, of which the outer layer is usually chitinous 
or calcareous, and constitutes a “cell” in which the zodid is 
contained. This outer layer is known as the “ectocyst,” to 
distinguish it from the ectoderm of the C@wlenterata. ‘The cell, 
thus formed, is lined by a much more delicate membranous 
layer, which is known as the “endocyst.” This membranous 
sac, formed by the endocyst, is pierced by two openings. One 
of these is the mouth, and it is always surrounded by a circle 
