PHYLUM ARTHROPODA 



621 



external appendages or gills (book-gills), in the shape of delicate 

 laminse attached to the abdominal appendages (Fig. 517). 



The nervous system is, in most instances, more concentrated 

 than in the Scorpions. There may be one or two separate 



hqb.du, 



FIG. 514. Diagrammatic view of a median longitudinal section of Limulus. abd.app. abdomina 

 appendages ; an. anus ; brn. brain ; chil. chilaria ; hep. du. opening of one of the hepatic 

 ducts ; ht. heart ; int. intestine ; 1. liv. liver ; mo. mouth ; ne. co. nerve cord ; oes. oesophagus ; 

 operc. operculum ; ids. telson ; ven. sinus, venous sinus ; 1-5, legs. (From Leuckart, partly 

 after Packard.) 



abdominal ganglia behind the mass formed by the united cephalo- 

 thoracic and anterior abdominal (Pseudoscorpionida, Pedipalpida, 

 some Araneida, Solpugida, Phalangida). In most of the Araneida 

 and in the Acarida all the abdominal are united with all the 



FIG. 515. Book-lung of a Spider 

 (Zilla callopkylla). */. 



stigma. (From Hertwig.) 



FIG. 516. Main branches of the tracheal 

 system of a Spider, st. stigma. 

 (From Hertwig, after Bertkau.) 



cephalothoracic ganglia to form a single mass perforated by the 

 oesophagus, the part lying behind, which is much the larger, 

 representing the ventral nerve cord. 



Sense organs. Eyes are present in all except in some of the 

 Acarida. Their number and arrangement have been given with 



