Last o’er the field the Mite enormous swims, Swells his red heart, and writhes his giant limbs. ERASMUS DARWIN, _The Temple of Nature_. PREFACE The Editors feel that they owe an apology and some explanation to the readers of _The Cambridge Natural History_ for the delay which has occurred in the issue of this, the fourth in proper order, but the last to appear of the ten volumes which compose the work. The delay has been due principally to the untimely death of Professor W. F. E. Weldon, who had undertaken to write the Section on the Crustacea. The Chapter on the Branchiopoda is all he actually left ready for publication, but it gives an indication of the thorough way in which he had intended to treat his subject. He had, however, superintended the preparation of a number of beautiful illustrations, which show that he had determined to use, in the main, first-hand knowledge. Many of these figures have been incorporated in the article by Mr. Geoffrey Smith, to whom the Editors wish to express their thanks for taking up, almost at a moment’s notice, the task which had dropped from his teacher’s hand. A further apology is due to the other contributors to this volume. Their contributions have been in type for many years, and owing to the inevitable delays indicated above they have been called upon to make old articles new, ever an ungrateful labour. The appearance of this volume completes the work the Editors embarked on some sixteen years ago. It coincides with the cessation of an almost daily intercourse since the time when they “came up” to Cambridge as freshmen in 1880. S. F. HARMER. A. E. SHIPLEY. _March 1909._ CONTENTS PAGE SCHEME OF THE CLASSIFICATION ADOPTED IN THIS VOLUME xi CRUSTACEA CHAPTER I CRUSTACEA GENERAL ORGANISATION 3 CHAPTER II CRUSTACEA (_continued_) ENTOMOSTRACA—BRANCHIOPODA—PHYLLOPODA—CLADOCERA—WATER-FLEAS 18 CHAPTER III CRUSTACEA ENTOMOSTRACA (_continued_) COPEPODA 55 CHAPTER IV CRUSTACEA ENTOMOSTRACA (_continued_) CIRRIPEDIA—PHENOMENA OF GROWTH AND SEX—OSTRACODA 79 CHAPTER V CRUSTACEA (_continued_)