Fleas are wingless members of the order, and their agility fully
compensates for the loss of the power of flight. There are many species,
infesting different mammals and birds. The females of the tiny
sand-fleas, or chiggers (_Sarcopsylla penetrans_), of America deposit
their eggs in the feet of human beings (or other animals), and unless
the painful swellings thus brought about are carefully treated they are
apt to fester dangerously.
=STRANGE ANIMAL FORMS FOUND IN THE SEA=
[Illustration: =SEA CUCUMBERS= (See Page 242)]
[Illustration: =SURFACE OF STARFISH= (See Page 242)]
[Illustration: =SNAKE-STAR= (See Page 242)]
[Illustration: =JELLY-FISH= (Page 243)]
[Illustration: =HEART-SHAPED SEA URCHIN= (See Page 242)]
[Illustration: =SEA ANEMONE= (See Page 243)]
[Illustration: =SEA URCHIN= (See Page 242)]
[Illustration: =BRANCHING CORAL= (Page 243)]
_STARFISHES AND SEA-URCHINS_ (_Echinoderms_)
=HEDGEHOG SKINNED ANIMALS OF THE SEA. SEA-LILIES, STAR FISHES, BRITTLE
STARS, SEA URCHINS, AND SEA CUCUMBERS=
Echinoderms, or hedgehog-skinned animals differ from all the forms
so far considered in the nature of their symmetry. Instead of being
two-sided, with a well-marked distinction between right and left and
front and back (bilateral symmetry), they resemble a star or regular
flower in shape. The skin is hardened by the deposition of salts of
lime, and the body is often covered with spines, as more
particularly in sea urchins.
Five existing subdivisions are recognized: (1) Sea-lilies and
feather-stars (_Crinoidea_); (2) Starfishes (_Asteroidea_); (3)
Brittle Stars (_Ophiuroidea_); (4) Sea urchins (_Echinoidea_); and
(5) Sea-cucumbers (_Holothuroidea_). All are marine.
=Sea Cucumbers= are sort of second cousins to the sea urchins and rather
more distantly related to the starfish. As the name indicates, they are
shaped like a cucumber, and hundreds of little feet on the side heighten
the resemblance, as they recall the spines on the vegetable. Inside they
have a coiled intestine, usually filled with mud and the contained
vegetable and other debris. With us no use is made of the animals, but
they are taken in great numbers in the South Seas, and sent to China,
where, as trepang, they form an ingredient in soups.
=Sea-lilies= are deep-sea animals, once numerous and flourishing, but
now comparatively rare, and only to be obtained by dredging in the
deeper parts of the ocean. They are fixed by a long, jointed stalk
bearing circlets of sensitive threads and terminating in a cup, in the
center of which the mouth is situated. Radiating from the edges of the
cup are five branching, feather-like arms, all of which are grooved
above, the grooves uniting, and finally converging to the mouth. They
are beset with cilia, and minute organisms are conducted inward along
them to serve as food.
=Sea Urchins= are radiated animals which are usually shaped like a
flattened sphere. They have a mouth, surrounded by five chisel-shaped
jaws at one pole, while the whole outer surface is covered with slender,
movable spines. Between the spines are numbers of slender, flexible,
tubular feet, which pull the body along, while the spines act more like
true feet. The animals feed mostly on seaweeds. They have no economic
value with us, but in Europe the eggs of some species are eaten, forming
part of the _frutti di mare_ of every Italian seaport.
=Starfishes.=--Starfishes are among the most familiar objects of the
seashore, and the commonest kinds, such as the five-finger (_Asterias
rubens_) and the comb-star (_Astropecten aurantiacus_) possess five
radiating arms. The mouth is in the center of the under side, and leads
into a capacious stomach, of which the first part can be protruded from
the body to surround such prey as mussels and oysters.
A starfish crawls slowly by means of numerous tube-feet, which are
lodged in five grooves radiating from the mouth, and make up a part of
the water-vascular system, so called because it is full of sea-water. At
the end of each arm is an unpaired tube-foot acting as a feeler, while
on its under side there is an orange-red eye-spot.
The water-vascular system assists in breathing. It was probably first
evolved in the interests of respiration, and this is its chief use in
the sea-lily. Some of the spines are formed of little, two-bladed
pincers, which clean the surface of the body.
Starfishes are remarkable for their powers of restoring lost parts. A
detached arm can grow a fresh disc and another four arms.
_ANIMALS THAT APPROACH THE SIMPLEST FORMS OF LIFE_