808 Marvels of the Universe 
two extremities, there is a single knot of nerves—its entire nervous system—and lower down there are 
muscles which attach the tube to the base of the body wall. These muscles are the machinery 
by which the mouth and tentacles are at will withdrawn into the cell. There is little else in the 
creature’s anatomy ; reproductive organs, of course, but no heart, no lungs, no liver, no eyes. 
This is a generalized account of the creature—which is spoken of as a polypide ; in the numerous 
groups into which the Polyzoa are broken up there are many variations of the details. The horny 
cell is really the polypide’s own outer skin, hardened for its protection. In some groups it is gela- 
tinous and soft ; in others it is impregnated with lime and becomes stony. From the coral-like 
appearance of the latter forms they were formerly called corallines, a name shared by stony-coated 
seaweeds and some of the Sea Firs. The Tertiary beds known as the Coralline Crag owe their name 
to the abundance of fossil forms of these 
lime-coated species, of which we give an 
illustration on page 811. 
The Sea Mat proper, whose English name 
has been applied as a popular designation of 
the whole group, is the pale brown, spongy- 
looking, though thin structure that branches 
like a seaweed, and is frequently thrown up 
on the sea-shore with the seaweeds that have 
been torn from the rocks by a rough sea. In 
this case it will be found to be made up of 
two layers of cells—or zocecia—cemented 
back to back. Other species form long, 
bristly threads that creep over the stems 
and fronds of seaweeds, and often when 
they are attached to the smaller weeds it is 
difficult to distinguish them from their host- 
plants. 
In the upper part of many of the zocecia 
there will be found a little chamber con- 
taining eggs, and by the embryos hatching 
from these new colonies are formed. The 
single zocecium resulting from one of these 
grows into a “mat” by the polypide 
Photo by] ~ tw.wer, throwing out buds on each side, and these 
THE BRISTLY DISH CORALLINE. becoming fully formed throw out other 
sboutan ich The seciped cll ane Se’ meer eneide PUGS, and So ad infinitum, each bud te- 
the field of the photograph. maining attached to its parent until the 
colony gets large enough to offer such resistance to the waves that it gets swept off by superior 
force and destroyed. 
The tentacles are hollow, and when they are extended the fluids of the body-cavity flow into 
them, and are thus brought into close contact with the water of the sea, from which oxygen is 
absorbed. They thus serve as gills. The openings of the zocecia can be closed by hinged doors 
or lids. 
In many of these Sea Mats there are certain cells of a remarkable character which must be men- 
tioned. They are smaller than the others and project beyond the general level ; they have thicker 
lids, and in some species they present a remarkable resemblance to the head of a minute bird, on 
which account they are known as avicularia. On exploring the cell we find there is little inside 
beyond the muscles that effect the opening and closing of the lids. | These are particularly prominent’ 
