32S 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



probably having the same function as in Scoiyio.^ The 

 stomach gives otf c^ecal prolongations which may extend far 

 into the limbs. There is usually a dilated short rectum, into 

 which the branched Malpighian ducts open. The nervous 

 system, more concentrated than that of the Arthrogastra^ is 

 reduced to a supra-cesophageal ganglion and a single post- 

 oesophageal mass, with four indentations on either side. There 

 are six or eight simple eyes in the anterior part of the cara- 

 pace. Auditory organs have not been discovered in these or 

 any other Arachnida. 



One of the most characteristic organs of the Aranei7ia is 

 the arachnidium^ or apparatus by which the fine silky threads 

 which constitute the web are produced. H. Meckel,^ who 

 has fully described this apj^aratus as it occurs in Epeira dia- 

 dema, states that, in the adult, more than a thousand glands, 



Fig. 89,A.—3f]/ffale Blondii (after BlancTiarc!).— The stomach with its cseca, and the 



remainrler of the alimentary caiiH] with the liver and Malpighian tubes. 

 Fig. 89, B.— The heart and arterial vessels of the same. 



with separate excretory ducts, secrete the viscid material, 

 which, when exposed to the air, hardens into silk. These 



1 Lyonet's " Anatonnie de diffcrentes Espece.s d'Insectes" ("Mem. du Mu- 

 seum d'Histoire Natnrelle," 1829) contains an elaborate account of this appa- 

 ratus, as well as of the structure of the pedipalps of the male spiders. 



2 " Mikroorraphie einisrer Driisenapparate der niederen Thiere." (Miiller's 

 " Archiv," 1846.) See also Buchholz and Landois. {Ibid., 1868.) 



