286 GLOSSARY. 



Oology. The study of eggs of birds. 



Ontogeny. The development of the individual from the germ- 

 cell to the mature form. 



Ontology. The study that relates to the being of an individual 

 organism. Formerly a synonym for metaphysics, 



Ocecia. Cells for receiving the ova of the Polyzoa. They are 

 also called ovicells and ovicysts. 



Operculum. (1) The gill-cover of fishes. (2) The disk closing 

 in the mouth of most univalve shells. 



Ophthalmite. The peduncle supporting the eye of the Decapod 

 Crustacea. 



Opistkocoelous. Said of vertebras which are concave only behind. 



Optic lobes or corpora quadrigemina (or bigemina). Oval or 

 spherical bodies giving rise, wholly or in part, to the optic 

 nerve. 



Optic thalami. The inferior ganglia of the cerebrum on which 

 the optic nerves rest. 



Organ-systems are the bone-system, nerve-system, &c. 



Orientation. The relative direction of parts. 



Orthognathous. When the jaws do not project and the teeth ai 

 perpendicular. The highest type of the Vertebrata. 



Oscula. The large exhalant apertures of a sponge. 



Ossification. The conversion of tissue into bone by the deposi- 

 tion of earthy matter. 



Osteine. The tissue of bone. 



Osteocomma or osteomere, bone-segment, sclerotome, or ver- 

 tebra. 



Ostioles. The smaller inhalant apertures of a sponge. 



Otoconium. The ear-dust of the higher Mammalia, composed of 

 calcareous particles. 



Otocyst. A vesicle, often containing otoliths, in some of the In- 

 vertebrata, and subservient to hearing. 



Otoliths. (1) The internal ear-bones of fishes. (2) Calcareous 

 bodies connected with hearing in many of the lower animals. 



Ovary. The organ in which the ova are produced. 



Ovicell, Ovicyst. See OoRcia. 



Ovipositor. A tubular organ possessed by many insects, and used 

 for placing the eggs ; it is a modification of the aculeus. 



