Branch Arthropoda 75 



of antennae, the smaller of which are called antennules. 

 The body is covered with a chitinous integument, a large 

 part of which is usually hardened with lime. This cover- 

 ing is cast off at frequent intervals during the growth of 

 the animal. During development many crustaceans pass 

 through a series of peculiar changes of form which are 

 called metamorphoses. Nearly all are water breathers ; 

 some live in fresh water or in moist situations on the 

 land, but the great majority are marine. 



The second class, Onychophora, includes only the single 

 remarkable genus Peripatus. The members of this genus 

 are peculiar creatures, which appear to be a connecting 

 link between the Annulata and the Arthropoda. Only 

 a few species are known, all inhabiting tropical regions. 



The members of the third class, Myriapoda, have a dis- 

 tinct head, but the segments of the thorax and abdomen 

 form a continuous series. The number of these segments 

 is indefinite. Each bears one or sometimes two pairs of 

 legs. They breathe air by means of tracheae. The centi- 

 pedes and millipedes or " thousand-legs " are typical rep- 

 resentatives of this class. 



The fourth class, Hexapoda, differs from the preceding 

 classes in having three distinct divisions of the body, — 

 head, thorax, and abdomen. The head is composed of several 

 united segments ; the thorax of three segments ; and the 

 abdomen of from seven to eleven segments. There are six 

 legs, one pair being borne by each thoracic segment. The 

 thoracic segments also bear on their dorsal surface one or 

 two pairs of wings. The nervous system has a wonder- 

 fully complex specialization, furnishing the insect with 

 great instinctive powers. Growth takes place only during 

 the early or larval life, and in many forms complicated 

 transformations occur. 



