GLOSSARY 237 



Micronucleus: the smaller of the nuclear bodies in infusorians. 



Mother-of-pearl: the inner layer of the shell of mollusks. 



Molt: to shed the cuticula or the outer portion of it. 



Mouth parts: the masticatory appendages on the head of arthropods. 



Mysis stage: a larval form of the lobster. 



Nauplius : a larval form of crustaceans. 



Nematocyst: the stinging organ in the Coelenterata which is within the 



cnidoblast. 

 Nephridium: a urinary tubule in anneHds. 



Nephrostome: the ciliated opening of a nephridium into the body cavity. 

 Nerve commissure: a nerve connecting the two members of a pair of 



gangUa. 

 Nerve connective: a nerve connecting two ganglia not of the same pair. 

 Nettle cell: the stinging organ in the Coelenterata. 

 Neuropodium: the ventral division of the parapodium of an annehd. 

 Nidamental glands: the large glands which secrete the egg capsules in 



the squid. 

 Notopodium: the dorsal division of the parapodium of an annelid. 

 Nucleus : a spheroidal body in a cell, the center of its activities. 



Ocellus: a minute, primitive eye. 



Ocular plate: the plate at the aboral end of a ray of the sea urchin. 



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



pharynx to the stomach. 

 Ommatidium: a single element of the compound eye of an arthropod. 

 Ooecium: a structure in Bryozoa in which the embryo develops. 

 Operculum: the flap covering the gill slits in fishes. 

 Oral: relating to the mouth. 



Oral groove: a groove leading to the mouth in ciliate infusorians. 

 Osculum: the cloacal opening in sponges. 

 Ossicles: the calcareous plates in the body wall of echinoderms. 

 Ovarioles: the tubules forming the ovary of an insect. 

 Ovary: the female sexual gland. 



Oviduct: the tube leading from the ovary toward the outside. 

 Ovipositor: the organ by means of which certain insects deposit their eggs. 

 Ovum: the female sexual cell, the egg. 



Pallial line: the line along which the margin of the mantle is attached to 



the shell in mollusks. 

 Pallial sinus: the indentation in the pallial line caused by the insertion of 



the siphonal retractor muscle. 

 Palp: a sensory organ near the mouth. 

 Pancreas: a digestive gland. 

 Papulae: the delicate projections of the body wall in the starfish. 



