222 PHYLUM ARTHROPODA 



The Cirripedia are scarcely recognisable as Crustacea apart 

 from their larval forms, which are more or less normal. 



SUBPHYLUM IV— MYRIAPODA 



These are terrestrial, and possess tracheae (p. 234) ; there is a 

 head with one pair of antennae, and many segments and legs. 

 They include two classes, which are not very closely related ; 

 the Chilopoda or centipedes, which are carnivorous, with a 

 flattened body and one pair of legs on each segment, and the 

 Diplopoda or millepedes, which are herbivorous with a cylindrical 

 body and two pairs of legs on each apparent segment, which 

 has been formed by the fusion of two somites. 



SUBPHYLUM V— INSECTA 

 This is discussed more fully below (p. 240). 



SUBPHYLUM VI— ARACHNIDA 



The body is divided into a prosoma of six somites, with 

 appendages of which four pairs are legs and none is an antenna 

 or mandible, and an opisthosoma of thirteen or fewer somites 

 which are often without appendages. The eighth segment bears 

 the genital opening, and the anterior part of the opisthosoma 

 bears respiratory openings of various types, including sometimes 

 the openings of tracheae. Traces of coelomoducts are present, 

 sometimes functional as excretory organs or genital ducts, as 

 in the scorpions. The development is nearly always direct. 



Most modern arachnids are terrestrial, and the best-known 

 forms in Britain are the class Araneida or spiders (Fig. 156). Here 

 the opisthosoma is large and sharply marked off from the prosoma 

 by a waist ; it has lost its segmentation and bears spinnerets, 

 which are transformed appendages, for guiding the silk which 

 is prepared by various glands. The first appendages are sub- 

 chelate chelicerae, and the second or pedipalps are modified in the 

 male to take up the semen which he has discharged on to 

 a leaf and convey it into the genital opening of the female. 

 Respiration is by lung-books, vascular plates situated in pockets 

 on the opisthosoma, and tracheae (Fig. 157). Apart from their 

 web-spinning, the most interesting thing about the spiders is 



