80 THE MICROSCOPICAL NEWS. 
tentacles, which may be expanded and retracted like pulling in and 
out the finger of a glove, but which are connected with the zocecitim 
firmly, so as to leave no opening between them. In most cases 
the alimentary canal or tube is furnished with muscles attached to 
the side of the zocecium, which can be expanded or retracted. The 
polyzoon contains no heart or vascular system. . The reproductive 
organs are contained in the cavity of the cell. 
The zocecium consists of an outer wall or ectocyst, and an inner 
wall or endocyst. The outer wall is a simple chitinose membrane, 
without any apparent structure, and is a secretion from the inner. 
wall or endocyst. This ectocyst is often strengthened by a deposit 
of calcareous or silicious particles, and then forms a solid wall, curi- 
ously and often very richly and beautifully sculptured. Sometimes 
it becomes gelatinous. The structure of this external cell is of 
great importance in systematic distinctions. The Polyzoa are social 
animals and live in colonies, which are formed by the production 
by gemmation of a number of these zocecia with the polypide con- 
tents. This process of gemmation continues until the vital energies 
are exhausted. A connection between the soft polypides of the 
cells is kept up by means of perforations in the wall of the zocecium, 
through which thread-like prolongations of the endosarc pass, and 
which serve to connect together all the members of a colony. The 
perforated portions of the ectocyst are called ‘communication 
plates.” In one branch of the polyzoa the communications are at 
the bottom of the cell, and proper circular perforations are made in 
the diaphragms separating the different cells. In JZ membranacea 
a curious modification of the ectocyst occurs, certain parts of every 
zocecium being flexible. By this means the organism is enabled to 
bend and adapt itself to the varying position of the seaweed upon 
whose fronds it is found. 
The endocyst is a delicate transparent membrane without any 
cellular structure. It is, in fact, a simple layer of protoplasm lining 
the ectocyst. Connected with the endocyst is an arrangement of 
muscles or fibres, which, by their contractions, are instrumental in 
compressing the membrane and reducing the dimensions of the 
cavity of the zocecium. These are called the parietal muscles. 
The zocecium is a more or less solid outer wall lined with living 
membrane, in which the polypide lives. The different parts of the 
polypide are, (I.) the tentacles and sheath; (II.) the alimentary 
canal; (III.) the nervous system ; (IV.) the muscles. 
I. The corona, or crown, is a bell-shaped wreath of tentacles 
borne on a circular stage, called the /ophophore, occupying the 
summit of the body and perforated by the mouth. The mouth is 
a simple orfice opening into the cesophagus, and so placed as to 
form the focus of the food whirled down by the action of the ten- 
tacles. The tentacles are hollow, closed at the extremities and 
