146 
NATURAL HISTORY. [NEW BUILDING. 
contractile skin. The marine kinds have the outer skin 
hard and horny, forming a sheath for the stomach and 
more important organs, and into which the head, the 
contractile tentacles, and other parts of the body can be 
withdrawn for protection. 
These animals live associated together in masses like 
the other Zoophytes, and their union appears to be even 
more intimate. In some, as the Tubularia , they assume 
the form of simple or slightly branched tubes, with the 
Polypi coming out at their ends. In the Sertularia , the 
animal throws out a number of lateral buds, which form 
for themselves small toothlike cases in the sides of the 
stem ; and as each species throws out its bud in a 
regular and definite manner, the united mass of animals 
resulting from this union assumes a defined form, wdnch 
is peculiar to each of the species. The latter animals 
also emit from different parts of their bodies variously 
shaped cells or vesicles which contain eggs, eventually 
emitted and becoming the origin of other similar masses 
of animals. 
The Class of Polyzoa (Cases 31 and 32) has many 
characters in common with the Zoophytes, with which 
it has very generally been confounded. The animals, 
which have also been called Bryozoa , are united together 
in similar common masses; but they are much more 
complicated in their organization, have a regular di¬ 
gestive canal, a separate stomach, and distinctf mouth 
and vent. Their mouth is surrounded with eight or 
more simple ciliated tentacles, which, when the animal 
is at rest, are contractile into the gullet. Their outer 
skin is in general thick or fieshy, and very often assumes 
a stone-like or glassy consistence. 
They are nearly allied to the compound Ascidici in 
their organization, but are distinguished from them 
by their mouth being furnished with distinct tentacles, 
and by their skin often gradually assuming a stony 
texture. 
In the fluviatile kinds, which are unisexual, and have 
a distinct ganglionic ring round the esophagus, have the 
series of tentacles interrupted on one side, so that they 
form a horse-shoe shaped group, as in Plumatella , which 
is found in the ponds and ditches near London. When 
