PHYLUM ARTHROPODA 



The pliyluu Artliiopoda is iUv largest of all phyhi, coniprisinj? 

 about four-fifths of all the known animal species. The funda- 

 mental plan of organization has enabled these animals to compete 

 successfully both with other animals and with the forces encoun- 

 tered in their environment. 



Class Crustacea Crayfish (or the Lobster May Be Used). — 

 With few exceptions the animals of the class Crustacea arc 

 aquatic. Respiration is either by means of gills or directly 

 through the surface of the body. 



The common crayfish is an inhabitant of fresh-water streams 

 and lakes. 



External Structure.— Study a specimen in a dissecting pan. 

 The hard external covering, the exoskeleton, is secreted by the 

 ectoderm and corresponds to the cuticle of the earthworm. It is 

 composed of chitin and serves for proiediou, support, and the 

 attaehment of muscles. As this covering cannot expand with the 

 growth of the animal, it must be molted (shed) periodically. 



Regional differentiation is pronounced as attested by the 

 presence of head, thorax, and abdomen in each of which the 

 component segments resemble each other more than they do 

 those of other regions. In the crayfish and others of its kind 

 the head and thorax are outwardly united. The line of demar- 

 cation is a groove over the sides and dorsum of the shell and 

 known as the rermcal or ncek groove. When head and thorax are 

 thus united it constitutes a eephaloihorax. The single piece of 

 exoskeleton covering this portion of the body dorsally and 

 laterally is known as the carapace. 



The head terminates in the anterior pointed extension of the 

 carapace called the rostrum. This division of the botly is pro- 

 vided with a number of sense organs more complex than those 

 found in forms previously considered. Notice the pair of large 

 compound eyes. These are stalked and movable and are con- 

 sidered by some to be appendages. In front of the eyes are the 

 smaller first sensory appendages or antennules. Just below are 

 the long, flexible, many-jointed pair, the antmiia'. Both pairs 



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