The structure of an Argulid is exhibited in Fig. 48. In front of the siphon, within which the styliform mandibles and first maxillae work, there is a poison-spine (_sp_); the appendages which correspond to the second maxillae (_mx_) are modified into sucking discs, but in the genus _Dolops_ they terminate in normal claws. The next pair of appendages, usually spoken of as maxillipedes (_mxp_), are clasping organs, and behind follow four pairs of thoracic swimming feet (1–4). The body is foliaceous, and they always apply themselves to their hosts with the long axis pointing forwards and parallel to that of the host, while on various parts of the under surface of the body are spines pointing backward which prevent the parasite being brushed off by the passage of the host through the water. These animals, alone among the Copepoda, possess compound eyes. A short sketch has now been given of the variations in Copepod organisation, but we cannot leave the subject without pointing out the rich field which still remains for the morphologist, especially in determining the true relationships of the parasitic families. CHAPTER IV CRUSTACEA (_CONTINUED_): CIRRIPEDIA—PHENOMENA OF GROWTH AND SEX— OSTRACODA =Order III. Cirripedia.= The Cirripedes are medium-sized Crustacea, with the body consisting of few segments, and enveloped in a mantle formed as a fold of the external integument, which may be strongly protected by calcified plates. The abdomen is greatly reduced. The larva, after hatching out as a Nauplius, and passing through a Cypris stage, when it resembles an Ostracod, fixes itself to a foreign object by means of the first antennae, and becomes a pupa, which after profound changes gives rise to the adult.