Fig. 854.—Crustacea. 1. Lobster (Astacus marinus); 2. Cray-fish (Astacus fluviatilis); 3. Crab (Cancer pagurus); 4. Shrimp (Crangon vulgaris); 5. Prawn (Palæmon serratus).

This class includes a number of familiar animals such as the barnacle, the crab, the lobster, shrimp, etc.; and curious as it may appear are closely related to our spiders. Their cases or coverings are all articulated or disposed in distinct segments. They breathe through gills or by tubes, and possess legs, or appendages for walking, eating, or guidance. They are generally marine creatures.

The shell of the crustacea is composed largely of lime, and of course becomes very hard in time. It is formed from the skin. The body, like that of an insect, is composed of head, thorax, and abdomen, divided into twenty-one segments, of which seven occupy the head, seven the thorax, and the remainder the abdomen. Twenty segments are furnished with legs, or feelers, or claws—a pair to a segment. The lobster or crayfish will give excellent examples of the anatomy of the macrura or lobster kind of crustacea. The heart is situated in the back.

The following table given by Professor Nicholson will explain the “segments and appendages” of the lobster:—

1stSegment,Eyes.
2nd“Lesser antennæ
3rd“Greater antennæ.
Head 4th“Pair of biting jaws.
5th“First pair of chewing jaws.
6th“Second pair of chewing jaws.
7th“First pair of foot jaws.
8th“Second pair of foot jaws.
9th“Third pair of foot jaws.
10th“First pair of legs (claws).
Thorax11th“Second pair of legs (small claws).
12th“Third pair of legs (small claws).
13th“Fourth pair of legs.
14th“Fifth pair of legs.
15th“Ground appendages.
16th“Swimmerets.
17th“Swimmerets.
Abdomen18th“Swimmerets.
19th“Swimmerets.
20th“Large swimmerets.
21st“No appendages (tail fin).

The tail is, as may be supposed, the great aid to locomotion in the lobster family, and they can swim backwards with great rapidity by its assistance. Lobsters shed their claws when alarmed, and are easily caught by a glittering bait.

The hermit crabs are interesting creatures, but do not possess the horny coat of the crab or lobster. They are therefore compelled to inhabit an empty shell, into which they thrust themselves, holding to the bottom of it by their tail, while a large claw guards the entrance. When the animal gets too big for his house he moves to another, leaving the old home for another hermit of the shore.

The crabs have no developed tails, and are therefore called brachyura—“short-tailed,” and they are walking creatures. There are king crabs, land crabs, and the common swimming crab. These animals can shed their shells as other crustaceans, and a curious fact is they shed them whole. How the claws come out must remain more or less a mystery. Réaumur investigated the action of the crayfish, and noticed that as the casting time approached the crustacea retired to some hiding-place and remained without eating. The shell becomes gradually loosened, and at last by putting its feet against a stone and pushing backwards the animal jerks himself away. It must be a painful operation, for the mill-like teeth of the stomach are also rejected, and the joints do not give way. After a while a new shell appears, and is cast in due time as before.