200 INVERTEBRATE ZOOLOGY 
Class 2. Chaetopoda. With sete, segmentally arranged. 
Order 1. Polychaeta. Mostly marine chaetopods with parapodia 
on which are numerous sete. Ex. Nereis. 
Order 2. Oligochaeta. Earthworms. Mostly freshwater or land 
chaetopods without parapodia and with few sete. Ex. Lumbricus. 
Class 3. Hirudinea. Leeches. Annelids with a sucker at each end 
and no appendages or sete. Ex. Hirudo. 
Class 4. Myzostomida. Disk-shaped parasites of echinoderms with 
five pairs of parapodia. Ex. Myzostoma. 
PHYLUM V. ARTHROPODA 
Externally segmented animals with segmented appendages. 
Class 1. Crustacea. Aquatic, gill-bearing arthropods; two pairs of 
antenne present. 
Division 1. Hntomostraca. Small, simply constructed crustaceans 
with a variable number of body-segments and without abdominal 
appendages. 
Order 1. Phyllopoda. Entomostracans with flat, leaf-like append- 
ages. 
Suborder 1. Branchiopoda. Elongated phyllopods with segmented 
body. Ex. Branchipus. 
Suborder 2. Cladocera. Laterally compressed phyllopods, the body 
of which is not distinctly segmented and is enclosed in a bivalve 
shell; second pair of antenne are swimming organs and _ project 
from the shell. Ex. Daphnia. 
Order 2. Copepoda. Elongated entomostracans with distinctly 
segmented body and without gills; the female often carries one or 
two egg-sacs. Ex. Cyclops. 
Order 3. Ostracoda. Minute, laterally compressed entomostracans 
with entire body enclosed in a bivalve shell. Ex. Cypris. 
Order 4. Cirripedia. Sessile, hermaphroditie entomostracans with 
body enclosed in a calcareous shell; barnacles. Ex. Lepas. 
Division 2. Malacostraca. Crustaceans with a constant number 
(20) of body-segments and nineteen pairs of appendages; abdominal 
appendages present. 
Subdivision 1. Phyllocarida. Primitive malacostracans with cara- 
pace and with leaf-like thoracic feet. Ex. Nebalia. 
