GLOSSARY 99 



from a fertilized egg in which the cells resulting from cleav- 

 age become arranged in two layers. 



Gamete — A reproductive cell. 



Invasion — A term used in ecology to donate the entrance of new 

 species into a habitat, usually due to changes produced by 

 species already present. 



Labium — The lower lip of an arthropod. 



Larva — A young, free-living animal which has not yet completed 

 its development. 



Lateral — Of or pertaining to the side of the body. 



Left — The lateral surfaces of any animal's body are determined 

 by placing the animal in a position comparable to the posi- 

 tion of the observer (i.e., with the anterior end uppermost 

 and with the dorsal surface toward the observer). Then 

 left and right of the animal correspond directly to left and 

 right of the observer. 



Longitudinal section — A section taken to include the main axis 

 of the body or parallel to the main axis. 



Madreporic plate — A finely perforated disc at the external open- 

 ing of the water vascular system of echinoderms serving to 

 keep out foreign matter. 



Matrix — A non-protoplasmic substance in which the cells of some 

 tissues are embedded. 



Metamorphosis — A marked change in an animal between the 

 time of hatching from the egg and acquiring adult bodj^ 

 form, involving the loss of some structures characteristic 

 of the larva and the acquisition of new structures charac- 

 teristic of the mature adult. 



Metazoa — "Many-celled animals;" all animals higher than the 

 Protozoa. 



Nymph — The young of those insects which at the time of hatch- 



