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 uephridium 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. 



Nidamental 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. 



(Esophagus : the gullet, the division of the digestive canal leading from the 

 pharynx to the stomach; grasshopper, 15; caterpillar, 21; crayfish or 

 lobster, 39 ; 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. 



Ocecium : 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. 



