THE LIFE OF CRUSTACEA
[CHAPTER I]
INTRODUCTORY
Everyone has some acquaintance with the animals that are grouped by naturalists under the name Crustacea. The edible Crabs, Lobsters, Prawns, and Shrimps, are at least superficially familiar, either as brought to the table or as displayed in the fishmonger's, and the most unobservant of seaside visitors must have had his attention attracted by living specimens of some of the more obtrusive species, such as the common Shore Crab. Many, however, will be surprised to learn that the Barnacles coating the rocks on the seashore, the Sand-hoppers of the beach, and the Woodlice of our gardens, are members of the same class. Still less is it suspected, by those who have not given special attention to the subject, that the living species of the group number many thousands, presenting strange diversities of structure and habits, and playing important parts in the general economy of Nature.
In addition to those just mentioned, a few Crustacea are sufficiently well known to be distinguished by popular names, such, for example, as Crayfish and Hermit Crabs, but for the vast majority no names are available except those of technical zoology. In the following pages, therefore, while technical terms have been introduced as sparingly as possible, the unfamiliarity of the animals themselves makes it needful to use many unfamiliar names.
In the classification of the Animal Kingdom, the Crustacea form one of the divisions of a comprehensive group, or Phylum, known as Arthropoda. The typical members of this group have a more or less firm external skeleton, the body is divided into segments, there are jointed limbs, and some of these are modified to serve as jaws. The chief divisions or classes of the Arthropoda are—(i.) Insecta, including Butterflies, Moths, Bees, Wasps, Flies, Beetles, and the like; (ii.) Chilopoda, or Centipedes; (iii.) Diplopoda, or Millipedes[1]; (iv.) Onychophora, including the curious worm-like Peripatus; (v.) Arachnida, or Scorpions, Spiders, Mites, and their allies; and (vi.) Crustacea.
It is not easy to summarize in a few words the characters common to all Crustacea, and distinguishing them from the other groups of Arthropoda. As a rough guide to classification, it is useful to remember that an Insect can generally be recognized by having three pairs of walking legs, an Arachnid by having four pairs, and a Centipede or Millipede by having a great many pairs, all nearly alike. The Crustacea, on the other hand, show great diversity in the number and arrangement of their walking or swimming legs, but they rarely show any special resemblance to those of the other large groups of Arthropoda. Thus, for example, a common species of Woodlouse, Armadillidium vulgare, is very similar at first sight to the Millipede Glomeris marginata, but it has only seven pairs of walking legs, while the Millipede has seventeen or nineteen pairs.
More precisely, it may be said of the great majority of Crustacea that they are aquatic animals, breathing by gills or by the general surface of the body, having two pairs of "feelers," or antennæ, on the front part of the head, and at least three pairs of jaws. Exceptions to each of these statements will be mentioned in later chapters in dealing with parasites and other highly modified types. In such cases, however, the larval or young stages afford indications of affinity, and comparison with less modified forms enables us to trace a connection with the typical Crustacea.