NATURAL HISTORY. 
133 
ROOM III.] 
being longer, more slender, and in having the side branches smaller. 
The Renillce. are fleshy, with a kidney-shaped expanded head, and the 
animals coming out on the broad flat sides ; and the Veretillum differs 
from the rest in being club-shaped, with the animals emitted on all the 
sides of the club. 
The Polypi aria, (Cases 29 and 30,) or third order of Zoophytes, 
like the Actinia , have numerous simple tentacles round their mouth, 
but the stomach is long, simple, and not, or only very slightly, longitu¬ 
dinally folded. 
The fresh water Polype, ( Hydra ,) which belongs to this group, is pe¬ 
culiar for being free and having a soft 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 con¬ 
tractile tentacles, and other parts of the body can be withdrawn for pro¬ 
tection. 
These animals live associated together in masses like the other Zoo¬ 
phytes, 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 them¬ 
selves small tooth-like 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, which is pe¬ 
culiar to each of the species. The latter animals also emit from dif¬ 
ferent parts of their bodies variously shaped cells or vesicles w 7 hich con¬ 
tain 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 JBryozoa , are 
united together in similar common masses; but they are much more 
complicated in their organization, have a regular digestive canal, a sepa¬ 
rate stomach, and distinct mouth and vent. Their mouth is surrounded 
with eight or mere simple ciliated tentacles, which, when the animal is 
at rest, are contractile into the gullet. Their outer skin is in gene¬ 
ral thick or fleshy, and very often assumes a stone-like or glassy con¬ 
sistence. 
They are nearly allied to the compound Ascidiam 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. 
The fluviatile kinds, which are unisexual, and have a distinct gan¬ 
glionic ring round the esophagus, have the series of tentacles interrupted 
on one side, so that they form a horse-shoe shaped group, as in Pluma- 
tella , which is found in the ponds and ditches near London. When 
attached to leaves which rot every year, the animal only forms a thin 
layer of branching tubes, but when attached to permanent bodies, as 
twigs, bricks, &c., they form thick masses. 
In the marine kinds, which are far more numerous, the tentacles form 
a continuous circular series. 
These animals live united together in masses, exhibiting almost all 
the different external forms assumed by the preceding Zoophytes. In 
