166 ZOOLOGY. 



Ichthyopsida (Gr. ichthus, a fish; opsis, appearance) The division 



of the Vertebrata which includes the Amphibia and fishes. 

 Ichthyosaurus (Gr. ichthus; saurus, a lizard) An extinct reptile 



found in the secondary rocks. 



Imago (Lat. an image) The perfect form of insects, 

 t^ Incisors (Lat. incido, I cut) The front or cutting teeth of 



mammals. 

 Incubation (Lat. incubo, I sit) The act of sitting on eggs, in order 



to develop the contained embryos. 

 Inequilateral (Lat. in, not; cequus, equal; latus, a side) Applied 



to the shells of the Lamellibranchiata, which have the two 



sides unequal. 



Inequivalve Having two unequal valves. 

 Infusoria (Lat. in, on ; /undo, I pour) A class of Protozoa, so 



called because they abound in infusions of animal or vegetable 



substances. 



Insecta (Lat. inseco, I cut into) A class of Arthropoda. 

 Invertebrata (Lat. in, not ; vertebra, a joint of the backbone) 



Animals which are destitute of a skull and spinal column. 



Labium (Lat. lip) The lower lip in the Arthropoda. 

 Labrum (Lat. lip) The upper lip in the Arthropoda. 

 ^^Labyrinthodon (Gr. labyrinthos, a maze; odous, tooth) An extinct 



amphibian, so called from the structure of its teeth. 

 Lamellibranchiata (Lat. lamella, a plate ; Gr. branchia, a gill) A 



class of Molhisca, so called because their gills are composed 



of folds of membrane. 

 Larva (Lat. a mask) The first stage of an insect after it comes 



from the egg. It is commonly called a caterpillar. 

 Lens (Lat. a bean) A term applied to the hexagonal facets into 



which the eyes of insects are divided. 

 Lobes Various rounded portions of animals are so called. 

 Lucernarida (Lat. lucerna, a lamp) A group of Hydrozoa. 

 \JLumbar (Lat. lumbus, the loin) Belonging to the loins. 



Madreporiform Applied to the tubercle by which the anibulacral 



system of the sea-urchins communicates with the exterior. 



It is so called because it is perforated with small holes like 



madrepore coral. 

 Mammalia (Lat. mamma, the breast) The class of Vertebrata 



which suckle their young. 

 Mandible (Lat. mando, I chew) The lower jaw in the Vertebrata; 



in Arthropoda, the upper pair of jaws ; the beak of the 



Cephalopoda. 



Mantle The outer covering of the Mollusca. 

 Manubrium (Lat. a handle) The handle-shaped polypite which is 



suspended from the centre of the disc in the Medusa* 



