Headless. The mantle consists of two flaps, which secrete two shelly
pieces (valves) movably united on the dorsal side (Fig. 43). The foot is
usually wedge-shaped. Respiration effected by gills placed in the
mantle-cavity, which communicates with the exterior either by the usual
respiratory opening, or by a breathing-tube (siphon). Most species are
marine, but some live in fresh water. No kind is of agricultural
importance. To this class belong the =Oyster= (_Ostrea edulis_), the
=Sea Mussel= (_Mytilus edulis_), the tropical =Pearl Mussel=
(_Meleagrina margaritifera_), the freshwater =Swan Mussel= (_Anodonta
cygnea_), the notorious =Ship Worm= (_Teredo navalis_), etc.
=Fifth Sub-Kingdom=: =ECHINODERMATA= (HEDGEHOG-SKINNED ANIMALS).
[Illustration:
FIG. 145.—The Common Starfish (_Asterias rubens_), seen on the right
from above, on the left from below.
]
The echinoderms are radially symmetrical, i.e. a number of similar parts
which together make up the body are arranged around an axis (Fig. 145).
These animals are usually five-rayed. The mouth lies at lower end of the
axis of the body. In some echinoderms the radial symmetry is indistinct,
and at the same time there is a more or less clear bilateral symmetry
(p. 16). The dermal skeleton either consists of numerous microscopic
calcareous deposits, or of firm plates; the skin often bears spines,
hence the name of the group. Echinoderms move by means of rows of
tubular feet, which are connected with a water-vascular system, and are
protruded by means of small bladders, connected with the same system,
which contract and force water into them. The form of the body is
apple-shaped to spherical (=Regular Sea Urchins=), shield- or
heart-shaped (=Irregular Sea Urchins=), pentagonal to star-shaped, with
arms of varying length (=Starfishes=), or cylindrical to vermiform (=Sea
Cucumbers=). All echinoderms are marine, and none of them, therefore,
are of agricultural importance, though near the coast starfishes are
sometimes used as manure.
=Sixth Sub-Kingdom=: =CŒLENTERATA= (ZOOPHYTES).
With a few exceptions the zoophytes are radially symmetrical (p. 252),
and four-, six-, eight-, or many-rayed. They consist of a body-wall
enclosing a cavity which, in the simplest forms (Fig. 146), is
sack-like, but in several of the higher sub-groups divided up, and
serves, not only for the reception and digestion of food, but also for
the circulation of the digested food. Many zoophytes develop hard parts
in the walls of their bodies, which remain after their death. The red
coral used for ornamental purposes is of this nature, and so also are
the coral islands which polypes build up. With the exception of the
sponges, which are usually reckoned as belonging to this sub-kingdom,
these animals possess “nettling organs” with which they kill their prey,
and by which they are protected from attack. They reproduce by eggs, and
most of them by means of budding as well (Fig. 146). In many species the
buds are not detached as in the Freshwater Polype (_Hydra_), but remain
attached so that a “colony” is formed. There are also free-swimming
cœlenterates, known as jellyfishes or medusæ (Fig. 147). Many of these
lay eggs which do not develop into fresh medusæ, but into polypes, which
increase by budding to form colonies, on which medusæ are once more
developed as buds. There is thus an _alternation of generations_ (p.
16), in which the medusæ represent the _sexual_ and the polypes the
_asexual_ stage.
[Illustration:
FIG. 146.—Freshwater Polype (_Hydra_), bearing two buds of different
ages. Highly magnified.
]
[Illustration:
FIG. 147.—A Phosphorescent Jellyfish (_Pelagia noctiluca_), ½ the
natural size.
]
[Illustration:
FIG. 148.—A Sea Anemone (_Sagartia nivea_).
]
Almost all zoophytes live in the sea, but some few in fresh water; none
are important agriculturally. The following belong to this sub-kingdom:
1. The =Hydroid Zoophytes= (_Hydrozoa_), including the medusæ of the
North Sea, many of which are the sexual stage in the life history of
fixed asexual polypes. The common =Freshwater Polypes= (_Hydra viridis_,
_fusca_, etc.). 2. The =Flower Polypes= (_Anthozoa_), to which belong
=Red Coral= (_Corallium rubrum_), and the =reef-building corals=
(_Astræa_, _Mæandrina_, etc.). 3. The =Sponges= (_Porifera_), which are
usually considered cœlenterates, though they possess no nettling organs
(=Freshwater Sponge= = _Spongilla_, =Bath Sponge= = _Euspongia
officinalis_).
=Seventh Sub-Kingdom: PROTOZOA= (ONE-CELLED ANIMALS).
[Illustration:
FIG. 149.—Two species of Proteus Animalcule (_Amœba_), one with
protoplasmic processes, slender and branched; the other with short
blunt processes. Highly magnified.
]
This sub-kingdom includes animals which are mostly microscopic, or at
any rate small, and live in the sea, fresh water, or damp places. They
are unicellular, and consist of a soft viscous substance (_protoplasm_
or _sarcode_) of contractile nature, and therefore capable of altering
its shape. They may or may not be enclosed by a firm wall. Many secrete
siliceous structures or calcareous shells. They are of no agricultural
importance. Here belong: =Infusoria=, common in putrid solutions; and
=Rhizopods=, such as _Amœba_, the =Proteus Animalcule= (Fig. 149), so
called because its shape is constantly altering.