778 CLASS V. ARACHNIDA. 



cavities of these two sinuses and a consequent inrush of blood 

 into both. 



The alimentary canal runs comparatively straight from the 

 mouth to the anus and is devoid of convolutions. Many Arach- 

 nids live on animal and vegetable juices and in these the mouth 

 is minute and a sucking pharynx, whose walls are divaricated 

 by extrinsic muscles, lies close within it. The alimentary canal 

 is different from that of the Crustacea since it gives off as a rule 

 more than one pair of digestive glands. The glands, of which 

 the anterior or prosomatic in the scorpion are termed salivary, 

 secrete a fluid which is believed to act like the pancreatic fluid of 

 vertebrates. The gastric glands are tubular in spiders and are 

 bent into the bases of the limbs. Their distal ends are said to 

 fuse in some species. They extend almost to the tip of some 

 of the limbs in the Pantopoda. There seems no doubt that the 

 food finds its way into the lumen of these glands and that a 

 large part of the digestion goes on in them and not in the cavity 

 of the alimentary canal. The proctodaeum varies in size ; mal- 

 pighian tubules (absent in Limulus) are one pair or more 

 numerous ; they open, as a rule, into the anterior end of the 

 hind-gut, which bears in the spiders an enlargement termed 

 the stercoral pocket in which the faeces accumulate. The 

 anus is not quite terminal and is always slightly ventral. It 

 may be succeeded by the sting as in scorpions or by a spine 

 as in Limulus. 



The excretion of waste nitrogenous matter is not confined 

 to the above-mentioned malpighian tubules. Coxal glands 

 are also present and are homologous with the green glands 

 and shell glands of Crustacea. They are tubular portions of 

 the coelom which open to the exterior. The opening in the 

 Arachnida is minute and for" long it escaped observation ; in 

 scorpions it is on the hinder face of the 3rd pair of legs, in Limulus 

 the gland gives off a lobe into the 2nd, 3rd, 4th, and 5th legs 

 and opens on the coxa of the 5th. In Atypus, one of the Araneina, 

 it opens both in the base of the first and third pair of legs. 



The internal skeleton or endosternite is cartilaginous in 

 texture but yields chitin instead of chondrin or gelatine. 

 This chitinous plate, which in Limulus " floats " between the 

 prosomatic nerve mass and the alimentary canal, and in the 

 scorpions forms a kind of diaphragm between the cavities of 



