CLASSIFICATION OF ARTHROPODS 219 



of which at least one pair serves as jaws, and a continuous 

 thickened chitinous cuticle. The classification of the phylum is 

 also difficult, on account of the vast number of species which 

 it includes. It is convenient, in order to get manageable units 

 at the lower end, to begin by a division into subphyla, although 

 these are probably more nearly the equivalent of classes in other 

 groups. 



SUBPHYLUM I— ONYCHOPHORA 



There are two genera, Peripatus (Fig. 151) and Peripatopsis. 

 They are aberrant forms, possibly not strictly arthropods at all, 

 possessing tracheae (p. 234) and cilia, and without either thick 

 cuticle or jointed limbs. 



Fig. 151. — Peripatus capensis, slightly enlarged. — From Borradaile, after 

 Sedgwick. Borradaile and Potts, The Invertebrata, 2nd edition, 1935. 

 Cambridge University Press. 



SUBPHYLUM II— TRILOBITA 



These are extinct forms, from the Palaeozoic era (p. 712), 

 which bear some resemblance to the next subphylum. 



SUBPHYLUM III— CRUSTACEA 



These are typical arthropods, nearly all aquatic in habit, so 

 that respiratory organs, if present at all, are usually gills. There 

 are two pairs of antennae, on the second and third somites, and 

 on the fourth somite there is a pair of mandibles. The limbs, of 

 which there is usually a fairly complete series, are reducible to 

 a type with two rami. There are not more than two pairs of 

 coelomoducts, both in the head. 



The typical larva is the characteristic nauplius, unsegmented, 

 but with three pairs of appendages and a median eye (Fig. 152). 

 The nauplius is however often absent or modified, while there 

 may be other quite different types of larvae. 



