280 



Arthropoda. 



(Spiders^ Pseudoscorpions, and others) must be specially noticed. 

 The nervous system is of the usual Arthropod type, but 



characterised in most forms by the 

 fact that all the ventral ganglia 

 are fused into a single mass ; a 

 series of distinct ganglia occurs in 

 quite a few {e.g. the Scorpions). 

 Of special sense organs only 

 the eyes mentioned above are 

 known ; but since some Arachnids 

 can produce sounds, it is very 

 probable that auditory organs are 

 also present. The alimentary 

 canal is characterised by the 

 presence of several c se c a arising 

 from the anterior portion of the 

 mesenteron and extending some 

 distance into the legs. In the 

 Spiders, a single large curved 

 CEecum arises on each side from 

 the mesenteron, is directed for- 

 wards and gives ofE branches which 

 enter the bases of the legs; the 

 front ends of the two casca lie close 

 together above the fore gut (Fig. 

 228), and in many Spiders, unite 

 at this point. Salivary glands are 

 present, and, unlike Insecta, many 

 Arachnida possess a large liver 

 consisting of numerous tubules 

 situated in the abdomen. In most Arachnida there are Malpighian 

 tubules, like those of the Insecta.* The respiratory organs 

 are represented either by tracheae, which open to the surface by 

 a small number of stigmata, or by so-called lungs ; the latter are 

 invaginations of the skin, each of which is again provided with 

 a series of flat evaginations, lying close together like the leaves 

 of a book ; each form of respiratory organ may occur alone, 

 or both may be present in the same individual. The vascular 

 system is often better developed than in Insects ; in the Scorpions 

 for instance, which are provided with lungs, there is a circula- 

 tion similar to that of many Crustacea; the blood flows from the 

 heart through a number of arteries ; the venous blood collects in 



Fig. 228. Alimentary canal of 

 a Spider, diagrammatic. b csecnm, 

 b' its anterior end, 6" lateral branches of 

 the same, into which the Malpighian 

 tubules open, I hepatic ducts, m mesen- 

 teron, o oesophagus, o' suctorial stomach, 

 u Malpighian tubule. — Orig. 



- * In a variety of Arachnida (Scorpions, Phalangiidffi, etc.) there is, in the cephalo- 

 thorax, a pair of large coxal glands which usually open at the base of the third, 

 pair of limbs. They have been regarded as excretory, and considered to be segmental 

 organs ; the correctness of this interpretation is doubtful. 



