PHYLUM ARTHROPOD A 205 



and paedogenetic habits in some groups. Polyembryony has 

 been demonstrated in some insects. Hermaphroditism is rare. 

 The centrolecithal egg undergoes partial, superficial cleavage in 

 most instances. So many different larval stages are involved in 

 the various groups of arthropods that the developmental cycle 

 will be considered separately for the individual classes of the 

 phylum. 



Classes. — There is much disagreement regarding the number 

 of classes into which the Arthropoda should be divided. Until 

 fairly recently, four or five classes were considered sufficient to 

 express the extent to which differentiation has proceeded in this 

 phylum. With the increase in our knowledge of the arthropods 

 no less than eleven or twelve groups merit recognition as classes. 

 Even a conservative judgment would demand that the Crustacea, 

 the Acerata, the Onycophora, the Diplopoda, the Chilopoda, the 

 Symphyla, the Myrientomata, and the Insecta be recognized 

 as classes. Some writers maintain that in addition to these, 

 several other groups such as the Pycnogonida, the Tardigrada, 

 the Linguatulida, and the Pauropoda should be admitted to the 

 rank of classes. 



Class Crustacea 



The cuticula of most crustaceans has become hardened 

 through the addition of carbonate and phosphate of lime to the 

 organic chitinous skeleton. Members of this class are typically 

 aquatic, though some (the sow bugs and land crabs) have become 

 modified for terrestrial existence. All modern representatives of 

 this class have two pairs of antennae. A carapace is very fre- 

 quently present. This has its origin as a fold at the head end and 

 grows backward as a continuous shield protecting the underl3nng 

 parts and obscuring the external evidences of segmentation of the 

 parts covered. In some instances, the entire body is enclosed in 

 this shell-like carapace (Fig. 99), which gives to these crustaceans 

 a confusing external resemblance to molluscs. More frequently, 

 the head and all or part of the thorax are covered by the carapace. 



Appendages. — All of the present-day crustaceans have two 

 pairs of antennae, though the trilobites (Fig. 96), all of which are 

 extinct, have but a single pair. The appendages are typically 

 Y-shaped and are said to be of the biramous or schizopodal type. 

 The stem of the Y, which provides attachment with the body 

 wall, is called the protopodite and is composed of two segments, 



