A horny or shelly plate developed, in certain Mollusca, upon the hinder part of the foot, and serving to close the aperture of the shell when the animal is retracted within it; also the lid of the shell of a Bala-nus or Acorn-shell; also the chain of flat bones which covers the gills in many fishes.


(Gr. ophis, a serpent). The order of Reptiles comprising the Snakes.


(Gr. ophis; batrachos, a frog). Sometimes applied to the order of Snake-like Amphibians comprising the Coecilioe.


(Gr. ophis; morphe, shape). The order of Amphibia comprising the Coecilioe.


(Gr. ophis, snake; oura, tail; eidos, form). An order of Echino-dermata comprising the Brittle-stars and Sand-stars.


(Gr. opisthen, behind; bragchia, gill). A division of Gasteropoda, in which the gills are placed on the posterior part of the body.


(Gr. opisthen, behind ; koilos, hollow). Applied to vertebrae the bodies of which are hollow or concave behind.


(Lat. os, mouth). Connected with the mouth.


(Gr. ornis, a bird; delphus, womb). The primary division of Mammals comprising the Monotremata.


(Gr. ornis, bird; skelos, leg). Applied by Huxley to the Deinosaurian Reptiles, together with the genus Compsognathus, on account of the bird-like characters of their hind-limbs.


(Gr. orthos, straight; keras, horn). A family of the Nautilidoe, in which the shell is straight, or nearly so.


(Gr. orthos, straight; pteron, wing). An order of Insects.


(Lat. diminutive of os, mouth). 1. The large apertures by which a sponge is perforated ("exhalant apertures"). 2. The suckers with which the Taeniada (Tape-worms and Cystic Worms) are provided.


(Lat. diminutive of os, bone). Literally small bones. Often used to designate any hard structures of small size, such as the calcareous plates in the integument of the Star-fishes.


(Gr. ostrakon, a shell). An order of small Crustaceans which are enclosed in bivalve shells.


(Gr. ous, ear; and lithos, stone). The calcareous bodies connected with the sense of hearing, even in its most rudimentary form.

Ovarian Vesicles or Capsules

The generative buds of the Sertularida.


(Ovarium). The organ by which ova are produced.


(Lat. ovum, an egg; aud pario, I bring forth). Applied to animals which bring forth eggs, in contradistinction to those which bring forth their young alive.


(Lat. ovum; and pono, I place). The organ possessed by some insects, by means of which the eggs are placed in a position suitable for their development.