SYNOPSIS OF THE FRESH-WATER PHYLLOPOD CRUSTACEA OF 



NORTH AMERICA. 



By a. S. Packard, Jr., M. D 



That group of Crustacea (crabs, lobster, sbrimps, water-fleas) known 

 as Phyllgjjoda^* is, so called from the leaf-like nature of the feet, which 

 are broad, flat, two or three lobed, and adapted for swimming and for 

 purposes of respiration rather than for walking. While the number of 

 feet in the lobster and crabs {Becapoda) is limited to five, in the phyllo- 

 pods the number ranges from ten to sixty, exclusive of the antennae and 

 jaws. The thoracic and abdominal regions are merged into each other, 

 and there is no special distinction in form of the appendages. One of 

 the most distinctive characters, however, in all except the highest family, 

 BrancMpodidw, is the large, loose carapace, which is attached b}^ a mus- 

 cle to the head, and elsewhere loosely covers the thorax and the base of 

 the abdomen ; in the lower forms {Estheriadce), this carapace becomes 

 enlarged, double, and folded on the side of the compressed body of the 

 auimal, protecting it as completely as a clam-shell its occupant; and so 

 striking is the resemblance to the bivalve-shells, especially the small 

 fresh water forms, such as Cyclas, that they are often mistaken for them 

 by collectors of shells. 



The lower forms, such as Lymnetis, have only ten pairs of swimming- 

 feet, with the body entirely enclosed by the shell, and are not much 

 higher in structure than the water-fleas {Gladocera), such as Dapnia 

 and Sida (Fig. 1), and are scarcely larger, beiug about a line in length. 



The eggs of the phyllopods are round, and protected by a hard shell, 

 sometimes, as in Limnadia, rough and polygonal. Why the eggshells are 

 so dense and tough will be seen below, when we speak of the singular 

 mode of life of these animals. The eggs are borne about by the females. 

 In Brancliipus and Artemia, they are carried in special egg-sacs, usually 

 long and slender, attached to the base of the abdomen. In Ajms, the 

 eggs are few in number, and contained in an orbicular sac, formed by 

 the adhesion of two of the circular lamellse of the eleventh pair of feet. 

 In the Limnadiadm there is no egg-sac, but the eggs are situated loosely 

 on top of the back, under the shell, and held in place by little filaments 

 arising from the legs. 



The young, when hatched, are more or less oval in form, and with but 

 two or three pairs of feet; the first two pairs of these feet representing 

 the antennae of the adult. The young is called a Nauplius, from its 

 resemblance to the young (Nauplius) of the copepodous Crustacea {En- 

 tomostraca). Fig. 2 a represents the Nc uplius of Brancliipus {Artemia 

 has a similar larva), and h that of Apus. The young of Limnadia has 

 but two pairs of appendages, with an enormous hypostoma, or upper 

 lip. 



The difference between the sexes is always well marked. In Branclii- 

 pus and Artemia, the second antennse are converted into large-clasping 



* From the Greek (j)V/i?Mv, leaf; novg, ttoWj, foot. 



