RADIATA.
Descending in the scale of animal organization, we come next to the
subkingdom RADIÁTA, or that in which the parts are arranged in a radiate
manner around a centre. Of this there are three classes,--the
ECHINODER´MATA (ἑχῖνος, hedgehog, δἑρμα, skin), containing the
Sea-urchins (_Echínus_), Starfishes, &c., in which the skin is furnished
with hard calcareous projecting spines or curiously formed imbedded
calcareous corpuscles, forming a rudimentary skeleton; the ACALÉPHÆ
(ἁκαλἡφη, a nettle), or Sea-nettles; and the POL´YPI (πολὑς, many, ποῦς,
foot), to which we shall confine our notice. It may be remarked that the
last two classes have recently been united to form the single class
CŒLENTERÁTA (κοῖλον, hollow, ἔντερον, intestine).
POLYPI.--These animals are mostly marine. They are either single (Pl.
XI. fig. 5), or compound (Pl. XI. fig. 15), _i. e._ the bodies are
united; in the latter case the bodies being usually situated in horny
cells upon a branched polypidom. But in many of them, which do not occur
in this country, there is an internal solid calcareous skeleton, of
which coral is an example. The animal bodies are soft, and furnished at
the front end with a crown of tentacles (fig. 15 _a_); these are
contractile, and serve to enable the animals to catch their prey. The
horny, branched, and plant-like polypidoms are often found on the
seashore, and are popularly confounded with sea-weeds.
_Hy´dra vulgáris_ (Pl. XI. fig. 5) is a fresh-water species, which is
commonly met with among collections of water-plants, and may generally
be obtained by collecting some of these and placing them in a glass jar
of fresh water. When the water has stood for some hours, the Polypes
will be seen, on careful examination, adhering to the sides of the
glass. The body of the animals is cylindrical, hollow, and furnished
with from six to ten tentacles, arranged in a circle, in the centre of
which is the mouth. The tentacles are hollow, and communicate with the
cavity of the body. On examination with a high power, the tentacles will
be found to exhibit minute oval sacs, containing a long fibre coiled up
within them; and when the tentacles are touched by any foreign body, the
fibres are suddenly discharged. These are the stinging or urticating
organs. The _Hydræ_ move very slowly; but the body is very contractile,
and is often seen of various forms. When a minute animal, as an
Entomostracan, happens to come into contact with the tentacles, these
curve around it, holding it firmly, and finally bringing it to the
mouth. It is then forced into the cavity of the body of the animal,
where it is digested, the remains being discharged at the mouth. The
movements of the _Hydra_, when devouring its prey, form a very curious
and interesting spectacle. The _Hydræ_ are propagated by budding or
gemmation, also by the formation of capsules in the walls of the body,
containing ova and spermatozoa. The young Polypes formed by budding are
represented in the figure, adhering to the base of the parent.
_Sertulária púmila_ (Pl. XI. fig. 15) is a marine species, the polypidom
being frequently found adhering to _Fuci_ and other sea-weeds; it is
about half an inch long. The cells are opposite, pointed at the ends,
and with an oblique orifice. The tentacles are fourteen in this species.
In the summer large ovate cells are found, arising from the polypidom;
these contain the eggs, and are called ovisacs or ovig´erous vesicles.