188 GLOSSARY. 



Maxilla. One of the mouth appendages of arthropods. 



Medusa. Jelly-fish; the sexual stage of certain hydroids. 



Membranells. Structures formed of fused cilia found in some Ciliata. 



Meropod. The fourth segment from the body of a crustacean leg. 



Mesenteric filament. The modified free edge of a mesentery of Actino- 

 zoa. 



Mesentery. A membrane that supports the intestine; one of the par- 

 titions of Actinozoa. 



Mesoglea. The jelly-like substance that separates the ectoderm and 

 endoderm of a coelenterate. 



Metamere. One of the serial body-segments of an animal, as in annelids. 



Metamorphosis. A change, especially in form, of an animal. 



Metapleural fold. One of a pair of folds on the sides of Amphioxus. 



Micronucleus. The smaller of the two nuclei of some Protozoa. 



Monilifonn. Resembling a string of beads. 



Monoecious. Sexes united in one individual. 



Mouth. The opening through which food is taken. 



Myoneme. A contractile fiber. See Vorticella. 



Nacre. The inner layer of a mollusk shell. 



Nematocyst. A weapon of a coelenterate; nettle cell. 



Nephridiopore. The external opening of a nephridium. 



Nephrostome. The inner opening of a nephridium. 



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



Nerve connective. A nerve connecting two ganglia not of a pair. 



Nettle cell. Nematocyst; a weapon of a ccelenterate. 



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



Nidamental gland. An accessory reproductive gland possessed bj' some 



females, especially gastropods and cephalopods. 

 Notochord. A supporting structure characteristic of Chordata. 

 Notopodium. The dorsal division of a parapodium of an annelid. 

 Nuchal groove. A groove in the neck. 

 Nucleus. An organ of a cell. 



Odontophore. A special structure in the mouth of most mollusca except 

 lamellibranchs. The name is applied to the whole structure, cartilage, 

 radula and muscles. (It is used by some authors as the equivalent 

 of radula.) 



Ocellus. A simple eye of an arthropod. 



Ocular plate. A plate at the end of an ambulacral area of an echino- 

 derm. 



Olfactory organ. An organ to distinguish odors. 



Ooecium. A structure in Polyzoa in which the embryo develops. 



