GLOSSARY. 495 



NoTOEEANCHiATA (Gr. notos, the back ; and hragcMa, gill). Carrying the 

 gills upon the back ; applied to a division of the Annelida. 



NoTOCHORD (Gr. notos, back; cliorde, string). A cellular rod which is devel- 

 oped in the embryo ofVertebrates immediately beneath the spinal cord, 

 and which is usually replaced in the adult by the vertebral column. Often 

 it is spoken of as the ' ' chorda dorsalis. " 



Ntjdibeanchiata (Lat. nudus, naked; and Gr. hragchia, gill). An order of 

 the Gasteropoda in whicli the gills are naked. 



NuMMULiNA (Lat. nummus, a coin). A genus of coin-shaped Foraminifers of 

 the Eocene period. Often spoken of as " Nummulites. " 



Occipital. Connected with the occiput, or the back part of the head. 



Oceanic. Applied to animals which inhabit the open ocean ( = pelagic). 



Ocelli (Lat. diminutive of oculus, eye). The simple eyes of many- Echino- 

 derms, Spiders, Crustaceans, Molluscs, &c. 



OcTOPODA (Gr. octo, eight ; j^ous, foot). The tribe of Cuttle-fishes with eight 

 arms attached to the head. 



OuoNTOCETi (Gr. odous, tooth; l-ctos, whale). The " toothed " Whales, in 

 contradistinction to the "whalebone " Whales. 



Odontoid (Gr. odous; eidos, form). The "odontoid process" is the centrum 

 or body of the first cervical vertebra (atlas). It is detached from the atlas, 

 and is usually anchylosed with the second cervical vertebra {axis), and it 

 forms the pivot upon which the head rotates. 



Odontophof.e (Gr. odous, tooth ; pliero, I carry). The so-called "tongue," 

 or masticatory apparatus of Gasteropoda, Pteropoda, and Ceplialopoda. 



(Esophagus. The gullet or tube leading from the mouth to the stomach. 



Oligoch^ta (Gr. oligos, few ; chaite, hair). An order of Annelida, compris- 

 ing the Earth-worms, in which there are few bristles. 



Omnivoeous (Lat. omnia, everything ; voro, I devour). Feeding indiscrimi- 

 nately upon all sorts of food. 



Operculata (Lat. operadum, a lid). A division of pulmonate Gasteropoda, 

 in which the shell is closed by an operculum. 



Operculum. 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 Balanus or 

 Acorn-shell ; also the chain of flat bones which cover the gills in many 

 fishes. 



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



Ophidobatrachia (Gr. op)Ms; batrctchos, a frog). Sometimes applied to the 

 order of Snake-like Amphibians comprising the Ccecilioi. 



Ophiomorpha (Gr. ophis; morphe, shape). The order oi Amphibia comi<rh- 

 ing the Ccecilice. 



Ophiuroidea (Gr. opJiis, snake ; oura, tail ; eidos, form). An order of Echino- 

 dermata, comprising the Brittle-stars and Sand-stars. 



Opisthobeanchiata (Gr. opistlien, behind ; bragchia, gill). A division of 

 Gasteropoda in which the gills are placed on the posterior part of the 

 body. 



Opisthoccelous (Gr, opistlien, behind ; koilos, hollow). Applied to vertebice, 

 the bodies of which are hollow or concave behind. 



Oral (Lat. as, mouth). Connected with the mouth. 



