Decolopodidae (p. 531). Colossendeidae = Pasithoidae (p. 532). Eurycididae = Ascorhynchidae (p. 533). Ammotheidae (p. 534). Rhynchothoracidae (p. 535) Nymphonidae (p. 536). Pallenidae (p. 537). Phoxichilidiidae (p. 538). Phoxichilidae (p. 539). Pycnogonidae (p. 539). CRUSTACEA CHAPTERS I and III-VII BY GEOFFREY SMITH, M.A. (OXON.) Fellow of New College, Oxford CHAPTER II BY THE LATE W. F. R. WELDON, M.A. (D.SC. OXON.) Formerly Fellow of St. John’s College, Cambridge, and Linacre Professor of Human and Comparative Anatomy, Oxford CHAPTER I CRUSTACEA—GENERAL ORGANISATION The Crustacea are almost exclusively aquatic animals, and they play a part in the waters of the world closely parallel to that which insects play on land. The majority are free-living, and gain their sustenance either as vegetable-feeders or by preying upon other animals, but a great number are scavengers, picking clean the carcasses and refuse that litter the ocean, just as maggots and other insects rid the land of its dead cumber. Similar to insects also is the great abundance of individuals which represent many of the species, especially in the colder seas, and the naturalist in the Arctic or Antarctic oceans has learnt to hang the carcasses of bears and seals over the side of the boat for a few days in order to have them picked absolutely clean by shoals of small Amphipods. It is said that these creatures, when crowded sufficiently, will even attack living fishes, and by sheer press of numbers impede their escape and devour them alive. Equally surprising are the shoals of minute Copepods which may discolour the ocean for many miles, an appearance well known to fishermen, who take profitable toll of the fishes that follow in their wake. Despite this massing together we look in vain for any elaborate social economy, or for the development of complex instincts among Crustacea, such as excite our admiration in many insects, and though many a crab or lobster is sufficiently uncanny in appearance to suggest unearthly wisdom, he keeps his intelligence rigidly to himself, encased in the impenetrable reserve of his armour and vindicated by the most powerful of pincers. It is chiefly in the variety of structure and in the multifarious phases of life-history that the interest of the Crustacea lies. Before entering into an examination of these matters, it will be well to take a general survey of Crustacean organisation, to consider the plan on which these animals are built, and the probable relation of this plan to others met with in the animal kingdom.