GLOSSARY 491 



Neuroptera (Gr. neuron, nerve, cord ; pteron, wing) : an order of the Insecta 

 having four membranous wings, strengthened by tendinous cords or 

 " nervures," e.g. dragon-flies. 



Notostraca (Gr. notos, back ; ostrakon, shell) : an order of the branchiopod 

 crustaceans comprising Apus and Lepidurus, so named on account of 

 the shell-like carapace which covers the dorsal surface of the head and 

 thorax. 



Nyctiphanes (Gr. nuctiphanes, visible by night) : a crustacean belonging to 

 the order Euphausiacea, having luminous organs or photospheria on 

 the basal joints of certain of the thoracic feet. 



Ocellus (L. diminutive of oculus) : the name given to a single or simple eye 

 as contrasted with a multiple or aggregate eye. The ocellus may be a 

 simple placode or eye-spot ; an optic pit, with open mouth ; or a closed 

 vesicle, having a corneagen or vitreous lens. In some cases a group of 

 larval ocelli are replaced in the adult insect (imago) by an aggregate or 

 compound eye. 



Ommatidium (Gr. omma, eye, diminutive suffix) : one of the component 

 units of an aggregate or compound eye of an adult arthropod, e.g. 

 Dytiscus marginalis. Each ommatidium consists of an outer dioptric 

 part, formed of the vitreous cells or crystalline-cones, and an inner 

 percipient part called the retinula, which is composed of a central 

 visual-rod or rhabdome, and the neuro-sensory cells. The retinula; 

 are separated by inter-retinular pigment cells. 



Oniscus : wood louse, a crustacean belonging to the order Isopoda. They 

 resemble the trilobites in being able to roll themselves up into a ball, and 

 in the feet being like one another and approximately equal in size. 



Ontogeny (Gr. on, ontos, a being or individual ; genesis, origin) : the life- 

 history of an individual, including embryonic development. 



Ordovician System or Period : the geological strata between the Silurian 

 and Cambrian found in Bala, Llandello, and Scotland. It contains 

 remains of marine invertebrates, e.g. small ostracode Crustacea. 



Osteolepis (Gr. osteon, bone ; lepis, scale) : an extinct Ganoid fish having a 

 protective covering of closely set bony scales, and a large pineal foramen 

 situated in a single frontal bone. See Lepidosteus. 



Ostracoda (Gr. ostrakon, shell) : an order of small bivalve Crustacea, including 

 Cypris, in which median and sometimes both median and lateral eyes 

 are present. 



Paltemon : prawn, belonging to the subclass Malacostraca, or soft-shelled 

 crustaceans. 



Palisade cells (Fr. palissade, enclose with fence) : the name given to the tall 

 columnar cells which form a single layer of cells beneath the cuticle 

 in many invertebrate animals. They are also termed " hypoderm 

 cells." 



Paraphysis (Gr. para, beside ; phusis, growth) : a branched tubular outgrowth 

 from the roof of the third ventricle. It lies in front of the epiphysis and 

 the postvelar arch or dorsal sac. It is well-developed in Sphenodon 

 and certain fishes. 



