802 



CLASS V. ARACHNIDA. 



C. On the metasoma there are no appendages. 

 Within the body there is an internal skeleton in the form of 

 the endosternite just as there is in Limulus and Mygale. It 

 takes the form of a complex, triangular plate pierced with holes 

 for the nerve-cord and for blood-vessels to traverse, and its 

 edges are produced into processes for the attachment of muscles. 

 Its texture is cartilaginous, though liistologically and chemically 

 it differs from vertebrate cartilage. It jaelds chitin instead of 

 gelatine. It lies obliquely in the body and roughly divides the 

 cavity of the prosoma from that of the mesosoma. Little is 



known about its origin or about its 

 function. There is a second small en- 

 dosternite ventral to the nerve-cord 

 in the segment which bears the pec- 

 tines. 



The mouth is minute as befits a 

 creature which takes only liquid food, 

 chiefly the blood of spiders or insects. 

 It opens into a suctorial pharynx with 

 elastic, chitinous walls. These walls 

 can be divaricated by certain extrinsic 

 muscles and thus the pharynx acts as a 

 suctorial stomach. From this an oeso- 

 phagus (Fig. 521) passes backward 

 and receives the paired ducts of two 

 salivary glands. Salivary glands are 

 usually associated with terrestrial life 

 and are not found in Limulus. The 

 straight intestine, into which the oeso- 

 phagus opens, runs without twist or 

 curve to the anus. During its course 

 it receives as many as five or six lateral 

 ducts w r hicli lead from as many lateral 

 gastric glands, often collectively termed 



the liver. The food passes from the narrow intestine into the 

 lumen of these glands and is there digested. At the anterior 

 end of the metasoma the intestine receives the openings of two 

 or four malpighian tubules engaged in excreting waste nitrogen- 

 ous matter. The occurrence of these organs again seems to be 

 favoured by a terrestrial mode of living and they have probably 



FIG. 521. Dorsal view of Bitthits 

 occitanw, the dorsal integu- 

 ment has been removed to 

 show the digestive organs 

 (from Leuekart, after Blan- 

 chard). 1 Chelicerae : 2 oeso- 

 phagus ; 3 salivary glands ; 

 4 intestine ; 5 liver ; 6 ducts 

 of liver : 7 malpighian tubules ; 

 8 intestine. 



