GLOSSARY 699 



Metasoa. The division of the animal kingdom comprising the many-celled 



animals. 

 Micronuclcus: The smaller of the nuclear l)odies in infusorians. 

 Microscleres. The minute spicules in sponges. 

 Minu'idium^ The youngest larval stage of trematodes. 

 Montj'cious. Having the two sexes united in one iiidivi(hial. 

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

 Moult. To shed the cuticida or the outer ])ortion of it. 

 Mouth-parts. The masticatory appendages on the head of arthropods. 



NaupHus. A young larval form of many crustaceans, especially entomostracans. 

 Nematocyst. The stinging organ in the Cnidaria which is within the cnidoblast. 

 Nephridium. A urinary tubule in annelids. 



Nephrostome. The ciliated opening of a nephridium into the body-cavity. 

 Nerve commissure. A nerve connecting the two members of a pair of ganglia. 

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

 Nettle cell. The stinging organ or nematocyst in the Cnidaria. 

 Neuropodium. The ventral division of the parapodium of an annelid. 

 Nidamental glands. The large glands which secrete the egg-capsules in the 



squid. 

 Notochord. A cylindrical rod formed in the young embryo along the mid-dorsal 



line of the digestive tube and ventral to the nervous system in ehordates. 

 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 in arthropods. 



Qj^sophagus. 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. 

 Onchosphere. The six-hooked embryo of tapeworms. 



Operculum. A plate closing an opening or covering some other structure. 

 Otrcium. A structure in Bryozoa in which the embryo develops. 

 Oral. The side of the body containing the mouth in a radiate animal. 

 Oral groove. A groove leading to the mouth in ciliate infusorians. 

 Organs of Cuvier. Glandular tubes extending from the rectum in holothurians. 

 Organ of Keher. An excretory organ in pelecypods. 

 Orthoneuraiis gastropods. Snails in which the pleuro-visceral nerve connectives 



are not crossed. 

 Osculum. The cloacal opening in sponges. 



Ossicles. The calcareous plates in the body-wall of echinoderms. 

 Otocyst. An organ of hearing. 



Otoporpae. Thickenings of the ectoderm at the lithocysts in Narcomedusae. 

 Outer lip. The portion of the lip of the aperture of a snail shell away from 



the axis. 

 Ovary. The female sexual gland. 

 Ovicell. A brood-chamber or ooecium in bryozoans. 

 Oviduct. The tube leading from the ovary towards the outside. 

 Ovipositor. The organ by means of which certain arthropods deposit their eggs. 

 Ovum. The female sexual cell, the egg. 



Pallets. Small calcareous plates in the siphons of Teredo. 



Pallial line. The line along which the margin of the mantle is attached to the 



shell in pelecypods. 

 Pallial sinus. The indentation in the pallial line, caused by t]:e insertion of 



the siphonal retractor muscle. 

 Palp. A sensory organ near the mouth. 



