Fresh-water genera.—_Polyphemus_, Müller, with no rudimentary exites on first three thoracic limbs. _Bythotrephes_, Leydig (Fig. 13), with no trace of processes on the outer sides of the limbs. Marine genera.—_Evadne_, Lovén, the head not separated by a constriction from the thorax. _Podon_, Lovén, with deep cervical constriction. [Illustration: FIG. 24.—_Leptodora hyalina_, × 6. Lake Bassenthwaite. _A._1, First antenna; _Car_, carapace; I, VI, first and sixth thoracic appendages. ] =Fam. 8. Leptodoridae=: six pairs of thoracic limbs, with no gnathobase. Only genus, _Leptodora_, Lilljeborg (Fig. 24), from fresh water. _Note._—For extra-European Cladocera consult Daday, “Microskopische Süsswassertiere aus Patagonien und Chili,” _Termés Füzetek_, xxv., 1902, p. 201; for Paraguay, _Bibliotheca Zoologica_, Heft 44; for Ceylon, _Termés Füzetek_, xxi., 1898; and for Australia, Sars, _Christiania Vidensk. Forhand._ 1885, No. 8, and 1888, No. 7; and _Arch. f. Math. og Naturvid._ xviii., 1896, No. 3, and xix., 1897, No. 1.—G. W. S. CHAPTER III CRUSTACEA (_CONTINUED_): COPEPODA =Order II. Copepoda.= The Copepods are small Crustacea, composed typically of about sixteen segments, in which the biramous type of limb predominates. They are devoid of a carapace. Development proceeds gradually by the addition posteriorly of segments to a Nauplius larval form. Paired compound eyes are absent, except in Branchiura, the adult retaining the simple eye of the Nauplius. In a typical Copepod, such as _Calanus hyperboreus_ (Fig. 25), we can distinguish the following segments with their appendages: a cephalothorax, carrying a pair of uniramous first antennae (_1^{st} Ant._); a pair of biramous second antennae (_2^{nd} Ant._); mandibles (_Md._) with biting gnathobases and a palp, and a pair of foliaceous first maxillae (_Mx.^1_). Two pairs of appendages follow, which were looked upon as the two branches of the second maxillae, but it is now certain that they represent two pairs of appendages, which may be called second maxillae (_Mx.^2_) and maxillipedes (_Mxp._) respectively. Behind these are five pairs of biramous swimming feet, the first pair (_Th.^1_) attached to the cephalothorax, the succeeding four pairs to four distinct thoracic somites. Behind the thorax is a clearly delimited abdomen composed of five segments, the first of which (_Abd.^1_) carries the genital opening, and the last a caudal furca.