94 ANIMAL FORMS 



66) ; the third (Myriapoda, meaning myriad-footed) em- 

 braces the centipeds and " thousand-legs " ; the fourth (In- 

 secta) contains the insects ; and the fifth (Arachnida) in- 

 cludes the" scorpions, spiders, and mites. 



94. The Crustacea. The number of species of crusta- 

 ceans is estimated to be about ten thousand, and while the 

 greater number of these are marine, many are found in 

 fresh water and a few occur on land. In size they range 

 from almost microscopic forms to the giant crabs and 

 lobsters. They differ also in shape to a remarkable degree, 

 but at the same time there is a decided resemblance through- 

 out the group, except in those species which have become 

 modified by a parasitic habit. The characteristic external 

 skeleton is invariably present, and gives evidence of the 

 deep internal segmentation of the body. In the simple 

 Crustacea this is very apparent, but in the higher forms it 

 is usually more or less obscured, owing to the fusion of some 

 of the different segments, especially those of the head, as in 

 the crayfish (Fig. 59). 



The class of the Crustacea is subdivided into two sub- 

 classes (Entomostraca and Malacostraca), the first containing 

 the fairy-shrimps (Brancliipus, Fig. 53) and their allies, the 

 copepods (such as Fig. 54), the barnacles (Fig. 55), and a 

 number of other species. In their organization all are com- 

 paratively simple, usually small, and the appendages show 

 relatively little specialization. The other subclass contains 

 the more highly developed and usually large-sized Crustacea, 

 among which are the shrimps, crayfishes, lobsters, crabs, 

 and a number of other forms. 



95. Some simple Crustacea. While the members of the 

 first subclass are minute and inconspicuous, several species 

 are often remarkably abundant in our small fresh-water 

 pools. Among these is the beautifully colored fairy-shrimp 

 (BrancMpus, Fig. 53), with greatly elongated body and 

 leaf-like appendages, whose relatively simple character leads 

 the zoologist to think that they are among the simplest 



