Crustacea, mostly of small size, the carapace protecting the head and thorax, or the body entirely naked. Feet numerous, never less than eight pairs, mostly foliaceous or leaf-like, branchial in function. The eyes sometimes confluent, sometimes distinct and sub-pedunculate. There are two horny mandibles without palps, and the first pair of feet are oar-like, with setiform terminal appendages. The remaining feet are branchial, and adapted for swimming. The Phyllo-pods undergo a metamorphosis, the youngest forms being
Fig. 146. - Phyllopoda. Fairy Shrimp (Chirocephalus, or Branchipus, diaphanus).
"Nauplii." In Nebalia (fig. 147, C), however, which is the only marine Phyllopod, "Zoea-stages" are superadded as well.
Fig. 147. - Morphology of Phyllopoda. A, Lepidurus Angassi, viewed dorsally. B, Under side of head of the same. C, Nebalia bipes, one side of the carapace being removed, so as to show the branchial feet. D, Branchipus stagnalis, female. E and F, Young stages of the same. G, A magnified specimen of Estheria, in its living state.
The Phyllopoda are chiefly interesting from their affinity to the extinct Trilobites. In the typical genera Limnadia and Apus the body is protected by a carapace, which is bivalve in the former and shield-like in the latter. In Limnadia the carapace covers the greater part of the body, and opens along the ventral margin. There are from eighteen to thirty pairs of membranaceous and respiratory feet. In Apus the carapace is clypei-form and covers a portion of the abdomen ; and there are sixty pairs of feet, of which all but the first pair are foliaceous. Apus is gregarious, freshwater in habit, and often found in great numbers in pools and ditches in Europe. The different species of Branchipus (figs. 146 and 147, D) have the transparent body unprotected by any carapace, and are found in ponds and swamps in various parts of the world. The various "Brine-shrimps" (Artemia) are found inhabiting the brine-pans in salt-works, or occur in salt-lakes in both hemispheres, being especially abundant in Great Salt Lake in Utah.
In Estheria (fig. 147, G) the body is protected by a bivalve, sub-ovate carapace, which is extremely like the shell of a Bivalve Mollusc, not only in shape and appearance, but also in having the valves joined at their beaks dorsally, and marked with concentric lines of growth. The species live in fresh or brackish water. In Nebalia (fig. 147, C), the only marine type of the order, there is a bivalved carapace, which is furnished with a beak or "rostrum," and the eyes are pedunculated. There are two pairs of antennae, and eight pairs of leaf-like respiratory feet, followed by a series of natatory feet. There is no metamorphosis. Nebalia has decided affinities with the Stomapods, and perhaps is not properly referable to the Phyllopoda.