Crustacea 59 



22 — Viewed dorsally armor unsymmetrical, of one piece. With two toes, equal, or the 

 shorter one over 1-3 as long as the longer (fig. 20) Diurella; with single toe, or with 



an additional inconspicuous shorter one (fig. 21) (Trichocerca) Rattulus 



— Viewed dorsally, armor symmetrical 23 



23 — Armor cleft dorsally. Armor covering only dorsal half of body, Diaschiza; armor 



well developed, enclosing body (fig. 23, 24) (Salpina) Mytilina 



— Armor not cleft dorsally 24 



24 — Armor of one more or less cylindrical piece, foot 3-segmented 25 



— Armor either of several pieces, or ventrally cleft 27 



25 — Two lateral eyes, armor flattened, not spined (Squatlnella) Stephanops 



— Only a median eye 26 



26 — With long dorsal spines, foot short (Polychaeta) Macrochaeta 



— Without long dorsal spines. Head armored, body armor strong (fig. 22) Dinocharis; 

 head with only weak cuticle, body armor thin Scandium 



27 — Foot not annulate, one to three segmented 28 



— Foot annulate, sometimes attached mid- ventrally 29 



28 — Armor smooth, much longer than broad Schizocerca 



— Armor, exclusive of spines, not much longer than broad. Foot one-segmented (fig. 30) 

 Brachionus; foot 3-segmented (fig. 32) (Platyias) Noteus 



2 — Head with chitin cap. Armor of two lateral plates, body compressed (fig. 27) Colurella; 



armor of dorsal and ventral plates, body depressed (Lepadella) Metopidia 



— Head not armored. Foot 3-segmented, two large toes (fig. 25) Euchlanis; foot 2-seg- 

 mented, one toe (fig. 26) Monostyla; foot one-segmented, two toes (Cathypna, 

 Distyla) Lecane 



CRUSTACEA 



The following table includes only the genera of the free swimming, fresh water 

 Entomostraca found in Eastern United States. An immature stage, known as Nauplius 

 is shown in fig. 3. 



1 — Without a shell like covering of the body; with four or five 2-branched swimming 



feet on the thorax, abdomen without appendages Copepoda — 2 



— Usually with a shell like covering which may or may not entirely cover the body; 

 if without shell the paired eyes are pedunculate 5 



2 — Cephalothorax distinctly separated from abdomen, the latter small 3 



— Cephalothorax not sharply differentiated; antennae short, at most 8-segmented — 

 (Harpactidae) ; with 8-segmented antennas (fig. 19) Canthocamptus 



3- — First antennae long, about as long as the body, 23-25 segmented; in the male, one is 



modified into a grasping organ; fifth feet not rudimentary. .(Cextropagidae) — 4 



— Antennae shorter than cephalothorax; female with 2 egg sacks (fig. 20) — (Cyclo- 



pidae) Cyclops 



4 — Endopodites of swimming feet of 3 segments. Antennae of 23 or 24 segments — 



Osphranticum; antennae of 25 segments (fig. 17) Limnocalanus 



— Endopodite of first swimming feet of one or two segments. Endopodite of first, 

 second, third and fourth swimming feet of one segment, Epischura; endopodite of 

 first swimming feet of two segments, of third and fourth feet of three segments 

 (fig. 16) Diaptomus 



5 — Trunk limbs leaf like in form; mandible without palp Phyllopoda — 6 



— Trunk limbs not leaf like; mandible with palpus; body not distinctly segmented 

 with caudal furca; antenna large, used for locomotion; bivalve shell, enclosing 

 entire body Ostracoda — 34 



