28o THE INVERTEBRATA 



canal near the junction of mid and hind gut in the Arachnida, Insecta, 

 and Myriapoda, In arachnids they are of endodermal origin, but in 

 insects and myriapods they are part of the ectodermal hind gut. It 

 is interesting that the subphyla differ in the nature of their nitro- 

 genous excreta. In the Crustacea these are principally ammonia 

 compounds and amines, in the Insecta they are urates, in the 

 Arachnida guanin. 



Nearly all the muscular tissue of arthropods is composed of striped 

 fibres, but in Peripatus only the fibres of the jaw muscles are striped, 

 and among the higher groups certain exceptions to the rule are 

 known (some visceral muscles, etc.). 



The gonads are always, owing to the reduction of the coelom, 

 directly continuous with their ducts, which are probably coelomoducts. 

 These have no constant position of opening in the phylum. In the 

 Crustacea they nearly always open at the hinder end of the thorax. 

 In the Arachnida their opening is similarly near the middle of the 

 body. In the Onychophora, Insecta, and centipedes they open near 

 the hinder end, but in the remaining groups of the Myriapoda their 

 opening is not far behind the head. 



The ova are generally yolky, and their cleavage is typically of the 

 kind known as "centrolecithal", in which (Fig. 202) the products 

 of division of the nucleus come to lie in a layer of protoplasm upon 

 the surface of a mass of yolk which thus occupies the position of a 

 blastocoele. The mode of gastrulation varies from invagination to 

 obscure processes of immigration and delamination. The formation 

 of the mesoblast as a pair of ventral bands, proliferated in primitive 

 cases from behind, has already been mentioned (p. 121). As in 

 annelids (p. 251), the mesoblast bands segment, and in most cases 

 the segments (" mesoblastic somites") develop coelomic cavities 

 (p. 279). In spite of the yolky eggs, there is a great variety of larval 

 stages, though direct development is also frequent. The series of 

 somites, which in the adult is often obscured by the loss, ob- 

 solescence, or fusion of some of its members, is usually more 

 distinct in the embryo or larva, where the presence of a somite 

 which it is difficult or impossible to recognize at a later stage is 

 frequently indicated by one or more of three criteria: a pair of 

 segments of mesoblast (mesoblastic somites), a pair of segmental 

 ganglia, and a pair of limbs or limb rudiments. 



