XXXII. PIECE OF TIMBER FROM RYDE PIER, SHOWING DAMAGE CAUSED BY _Limnoria_ AND _Chelura_. (_From Brit. Mus. Guide_) 255 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_. [1] The Chilopoda and Diplopoda are sometimes regarded as forming a single class--Myriopoda.