3l6 THE INVERTEBRATA 



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 coelomo- 

 ducts. 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. 



Typical features of the embryonic development are shown in Figs. 

 213, 316, and 352. The ova are generally yolky, and their cleavage 

 is typically of the kind knownas"centrolecithal",in which (Fig. 213) 

 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 (Fig. 213 D) to obscure processes of immigration and 

 delamination. The formation of the mesoblast as a pair of ventral 

 bands (Fig. 316), proliferated in primitive cases from behind, has 

 already been mentioned (p. 130). As in annelids (p. 285), the meso- 

 blast bands segment, and in most cases the segments (" mesoblastic 

 somites") develop coelomic cavities (Fig. 352). The haemocoele 

 arises by separation of the germ layers. The heart is formed by the 

 dorsal ends of the mesoblast segments approximating. The nerve 

 cords are proliferated from the ventral ectoderm (Fig. 352 A). 

 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, obsolescence, 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 im- 

 possible 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. 



