Fig. 65.—Segments of Millepede (magnified).
CHAPTER V
CRUSTACEANS.—PRAWN, SHRIMP, MYSIS, CRABS; AMPHIPODS; ISOPODS
The next group of Arthropod animals with which we have to deal is that of the Crusta´cea. Some or other of the members of this class are well known to everybody, if only in the shape of toothsome food—the Prawn, the Shrimp, the Lobster, the Crayfish, and the Crab. The great characteristic of this class of the Arthropod phylum is the so-called ‘shell,’ which differs greatly from true shell in being composed of chitine, hardened with salts of lime. Most of the species live in the water and breathe by means of gills or through the skin. In dealing with these creatures, some long words must be employed, if our present work on them is to be a stepping-stone to something more advanced. The difficulty is more apparent than real, and if boldly faced will soon be overcome.
Our first division, or sub-class, of the Crusta´cea is that of the Malacos´traca, or animals with soft shells—a name originally adopted, as Mr. Stebbing tells us[42], ‘to distinguish such creatures as crabs and crawfish and prawns from such others as oysters and clams; not because of the absolute, but because of the comparative softness of their shells.’ Under this sub-class are grouped two Orders—the Stalked-eyed and the Sessile-eyed Crustaceans, the technical names for which are the Podophthal´ma and the Edriophthal´ma.
To the Stalked-eyed Crustaceans belong the Prawn, the Shrimp, Mysis, or the Opossum Shrimp, and the Crabs, to mention only those forms with which we are dealing here. The reason for scientific and popular names will be evident if living or spirit specimens are examined, for it will be seen that the eyes are elevated on stalks. Mr. Stebbing[43] relates an amusing story of a very intelligent student, who, on being told that the eyes (of the shrimp) were stalked, candidly confessed to having always thought that this appearance was due to their having been forced out of the head by boiling.
The general shape of a Prawn (Palaemon serratus) is fairly familiar to everybody. The body is divided into two principal regions—the carapace, or cephalothorax, as it used to be called (formed by the union of the head and thorax), and the pleon, or swimming part. The carapace has a projecting beak or rostrum, and is unsegmented; the pleon is divided into segments, and the whole may be represented thus:—
where the long stroke (c) stands for the carapace, the shorter ones (1–6) for the segments of the pleon, and the ) for the telson or tail. The carapace consists of fourteen united segments, and this will give twenty or twenty-one segments in all, according as we reckon the telson an appendage of the sixth segment of the abdomen, or as a distinct segment. The carapace bears the eyes, two pairs of antennae, six pairs of mouth appendages, and five pairs of walking legs or perei´opods, normally with seven joints—in all, fourteen pairs of appendages, that is, one pair for each of the fourteen segments of which the carapace is composed. The segments 1–5 of the ple´on bear swimming feet, or ple´opods, and the female uses these for retaining the eggs, which she bears about with her. In this fashion the ‘hen’ lobster carries her ‘berries.’