ROOM III.] NATURAL HISTORY. 73 
only a single axis, which pervades their central stem, 
but they live floating free in the sea or with the naked 
part of the base of their stem sunk into the sand and 
mud on the coast, as the Hyalonema is into a sponge, 
and they are easily known from both by their symmetri¬ 
cal form, which, in the true Pennatula, resembles that of 
a pen with the animals coming out from the upper part of 
the side branches. Virgularia differs from the former in 
the stem being longer, more slender, and in having the side 
branches smaller. The Renilloe 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 Actinice, have numerous simple ten¬ 
tacles round their mouth, but the stomach is long, simple, 
and not, or only very slightly, longitudinally folded. 
The fresh water Polype, ( Hydra ,) which belongs to 
this group, is peculiar 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 
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withdrawn for protection.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 manper, the united mass of animals 
resulting from this union assumes a defined form, w r hich 
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 
