flexible, its basal joint being longer than the others, and bearing on its outer side a large, triangular, membranous appendage, and four soft cylindrical appendages, the main stem and its appendages being beset with curious tubercles, ending in short spines, whose structure is not understood. Except during the act of copulation this remarkable apparatus is coiled on the inner side of the antennary claw, the jointed stem being so coiled that it is often compared to the coiled proboscis of a butterfly, and the triangular membrane folded like a fan beside it, so that much of the organ is concealed, and the general appearance of the head is that shown in Fig. 5. During copulation, the whole structure is widely extended. [Illustration: FIG. 7.—_Artemia fertilis._ Front view of the head of a male, showing the large second antennae, _A.2_; _A.1_, first antennae. ] The males of _Artemia_ (Fig. 7) have the second antenna two-jointed, the basal joint bearing an inner tubercle, the terminal joint being flattened and bluntly pointed, its outer margin provided with a membranous outgrowth. In _A. fertilis_ the breadth of the second joint varies greatly, the narrower forms presenting a certain remote resemblance to _Branchinecta_. In the males of _Polyartemia_ the second antennae have a remarkable branched form not easily comparable with that found in other Branchipodidae. The cephalic jaws are fairly uniform throughout the order. The mandibles have an undivided molar surface, and no palp; the first maxilla is very generally a triangular plate, with a setose biting edge; mandibles and maxillae are covered by the labrum. The second maxilla generally lies outside the chamber formed by the labrum, and is a simple oval plate, with or without a special process for the duct of the kidney.