90 LABORATORY DIRECTIONS 



Gastrula That stage in the development of a new individual 

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

 age become arranged in two layers. 



Invasion A term used in ecology to denote 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 body form, 

 involving the loss of some structures characteristic of the 

 larva and the acquisition of new structures characteristic 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- 

 ing resemble the adult except for the lack of certain adult 



