216 INVERTEBRATE ZOOLOGY 
Mouth-parts: the masticatory appendages on the head of arthropods; wasp, 
2; grasshopper, 12; beetle, 13; wasp, 13; caterpillar, 20; centiped, 
22; crayfish or lobster, 30; crab, 43. 
Mysis stage: a larval form of the lobster, 51. 
Nauplius: a larval form of crustaceans, 59. 
Nematocyst: the stinging organ in the Cnidaria which is within the cnido- 
blast; Hydra, 161; tubularian, 165; campanularian, 171. 
Nephridium: a urinary tubule in annelids; Nereis, 65; earthworm, 73. 
Nephrostome: the ciliated opening of a nephridium into the body-cavity ; 
Nereis, 65 ; earthworm, 73. 
Nerve commissure: a nerve connecting the two members of a pair of gan- 
glia; planarian, 78; tapeworm, 82; mussel, 97; clam, 111; snail, 121. 
Nerve connective: a nerve connecting two ganglia not of the same pair; 
grasshopper, 18; spider, 27; crayfish or lobster, 41; Nereis, 65; mus- 
sel, 97; oyster, 102; clam, 111. 
Nettle cell: the stinging organ in the Cnidaria; Hydra, 159; Gonionemus, 
176; sea anemone, 178. 
Neuropodium : the ventral division of the parapodium of an annelid, 63. 
Widamental glands: the large glands which secrete the egg-capsules in 
the squid, 133. 
Notopodium : the dorsal division of the parapodium of an annelid, 63. 
Nucleus: a spheroidal body in a cell, the center of its activities ; Parame- 
cium, 186; Vorticella, 190; Euglena, 193; Amoeba, 195, 
Ocellus: a minute primitive eye; wasp, 2; fly, 7; grasshopper, 9; cater- 
pillar, 20; tubularian medusa, 168. 
Ocular plate: the plate at the aboral end of a ray of the sea urchin, 151. 
Csophagus: the gullet, the division of the digestive canal leading from the 
pharynx to the stomach; grasshopper, 15; caterpillar, 21; crayfish or 
lobster, 89; Nereis, 64; earthworm, 71; Bugula, 86; mussel, 96; 
clam, 110; snail, 117; squid, 130; Molgula, 137; starfish, 145; sea 
urchin, 151; sea cucumber, 156. 
Ommatidium : a single element of the compound eye of an arthropod. 
Occium: a structure in Bryozoa in which the embryo develops, 88. 
Oral: the side of the body containing the mouth in a radiate animal; star- 
fish, 142; sea urchin, 149; sea cucumber, 155; medusa, 167, 173, 175. 
Oral groove: a groove leading to the mouth in ciliate infusorians, 184, 188. 
Organ of Keber: an organ probably excretory in function in lamellibranchs ; 
mussel, 92; clam, 107. 
