GLOSSARY 261 



Odontophore. A special structure in the mouth of most mollusca ex- 

 cept pelecypods. The name is applied to the whole structure, car- 

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

 equivalent of radula.) 



Ocellus. A simple eye of an arthropod. 



Ocular plate. In echinoderms, a plate at the end of an ambulacral 

 area. 



Olfactory organ. An organ to distinguish odors. 



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



Ootype. The region in flat worms where the eggs are supplied with 

 shells. 



Operculum. The horny lid which fits into the aperture of the shell of 

 some gastropods. A chitinous protective structure on the hydro- 

 theca of certain hydroids which closes the hydrothecal aperture 

 when the hydranth is retracted. 



Oral. Pertaining to the mouth. 



Organelle. A specialized part of a cell, as a cirrus, performing func- 

 tions analogous to those of the organs of metazoa. 



Osculum. The opening of a sponge through which water escapes. 



Osphradium. A structure in the gill region of aquatic molluscs and 

 supposedly of sensory function. 



Ossicle. A small hard plate. 



Ostium. A small pore; in pelecypod gills, one of the pores through 

 which water is passed. In sponges, the opening through which 

 water enters an incurrent canal. 



Otocyst. A statocyst. 



Otolith. Statolith. 



Ovary. A gonad which produces eggs. 



Oviducal gland. A glandular portion of an oviduct, as in the squid. 



Oviduct. In a female, a duct which carries eggs to the uterus or the 

 exterior. 



Ovipositor. In some insects, an elongate structure used in depositing 



eggs. 

 Ovum. Egg. 



Pallial line. The depression in the shell of a pelecypod at the point 



of attachment of pallial muscles. 

 Pallial sinus. The indentation in the pallial line of some pelecypods 



at the point of insertion of the retractor muscles of the siphons. 

 Palp. A process near the mouth. It occurs in several invertebrate 



groups: e. g., in polychaete annelids, one of a pair of processes on 



the ventral side of the prostomium; in pelecypods, one of four 



fleshy processes around the mouth. 



