Fig. [6].—Chirocephalus diaphanus. Second antenna of male, uncoiled.
Of the cephalic appendages, the first antennae are generally small, and are never biramous; in Branchipus and its allies they are simple unjointed rods, in some species of Artemia they are three-jointed, in Apus they are feebly divided into two joints, while in Estheria they are many-jointed. The second antennae are the principal organs of locomotion in the Limnadiidae, where they are large and biramous; in all other Phyllopoda they are uniramous in the female, being either unjointed triangular plates as in Chirocephalus (Fig. [2]), or minute vestigial filaments as in Apus, in which genus Zaddach, Huxley, and Claus have all failed to find any trace of a second antenna in some females. In the male Branchipodidae the second antennae are modified to form claspers, by which the female is seized, the various degrees of complication which these claspers exhibit affording convenient generic characters. In Branchinecta each second antenna is a thick, three-jointed rod, the last joint forming a claw, while the second joint is serrate on its inner margin; in Branchipus the base is much thickened, and bears on its inner side a large filament (perhaps represented by the proximal tubercle of Branchinecta and Artemia), which looks like an extra antenna. In Streptocephalus the terminal joint of the antenna is bifid, and there is a basal filament like that of Branchipus; in Chirocephalus diaphanus (Figs. [5], 6) the main branch of the antenna consists of two large joints, the terminal joint being a strong claw with a serrated process at its base, while the proximal joint bears two appendages on its inner side; one of these is a small, subconical tubercle, the second is more complicated, consisting of a main stem and five outgrowths. The main stem is many-jointed and 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.
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.
The thoracic limbs, in front of the genital segments, are not as a rule differentiated into anterior maxillipedes and posterior locomotive appendages, as in higher forms; we have seen, however, that all these limbs take part in the prehension of food, and except in the Limnadiidae they all assist in locomotion. One of the middle thoracic legs of Artemia (Fig. [8], A) has a flattened stem, with seven processes on its inner, and two on its outer margin. The gnathobase (gn) is large, and fringed with long plumose setae, each of which is jointed; this is followed by four smaller “endites” (or processes on the median side), and then by two larger ones, the terminal endite (the sixth, excluding the gnathobase) being very mobile and attached to the main stem by a definite joint. On the outer side are two processes; a proximal “bract,” a flat plate with crenate edges, partly divided by a constriction into two, and a distal process, cylindrical and vascular, called by Sars and others the “epipodite.” In other Branchipodidae we have essentially the same condition, except that the fifth endite often becomes much larger than in Artemia, throwing the terminal endite well over to the outer edge of the limb; such a shift as this, continued farther, might well lead to the condition found in the Limnadiidae, or Apodidae, where the lobe which seems to represent the terminal endite of Artemia is entirely on the outer border of the limb, forming what most writers have called the exopodite (Lankester’s “flabellum”).[[18]] In the two last-named families the basal exite or bract of the Branchipodidae does not appear to be represented.
Fig. [8].—A, Thoracic limb of Chirocephalus diaphanus; B, prehensile thoracic limb of male Estheria. gn, Gnathobase; 1–6, the more distal endites.