ARACHNIDES. 191 



out apparent divisions, the envelope being a kind of bag or sac, 

 including the organs of circulation, respiration, the intestines, 

 and the secreting vessels of the matter which forms their web. 

 The heart is a large vessel running along the back, with branches 

 on each side. The respiratory organs, two in number, and com- 

 posed of minute laminae, are contained in the interior walls of 

 two sacs, situated at the lower part of the belly, one on each 

 side, and covered by a membranous operculum. A transverse 

 cleft affords a passage for the external air, and two yellowish or 

 whitish spots generally indicate the place of these organs. The 

 intestinal canal is short, with two dilatations, the last surrounded 

 by the liver. The vessels containing the matter of the web, 

 generally six in number, extend on each side interiorly, are of a 

 tortuous form, narrowed abruptly towards their extremity, and 

 terminate in a straight filament ending at the membranous pa- 

 pillae from which the threads are exuded. The generative 

 organs of both sexes are placed at the base of the belly, and are 

 double in all the pulmonary Arachnides. 



Some of the Arachnides live on land ; others in the water ; 

 and a third group are parasitical on different animals. In gene- 

 ral they are carnivorous, and suck the blood of their prey or 

 animals. A small number only feed on vegetable matters. 

 Many have mandibles which exercise the office of a sucker ; and 

 others have an isolated sucker, often, however, joined with man- 

 dibles and palpi. The terrestrial species are in general solitary 

 animals, and of a forbidding aspect, and many of them shun the 

 light, and live in concealment. Several of these are poisonous, 

 and their bite dangerous. 



Lamarck divided the class of Arachnides into three orders, viz. 

 1. Those destitute of antennae, furnished with branchial sacs for 

 respiration, and with six to eight eyes ; 2. Those destitute of 

 antennae, with branched trachea for respiration, and with two or 

 four smooth eyes ; 3. Those with antennae and gangliated trachea 

 for respiration : while Latreille arranges the class into two or- 

 ders, according to the characters of their branchial apparatus. 



Order I. PULMONARY. With pulmonary sacs for respiration ; 

 a heart and distinct vessels. 



Order II. TRACHEARLE. Respiring by tracheae, and the or- 

 gans of circulation indistinct. 



