ARACHNIDES. 161 



insects, are commonly terminated by two hooks, and even 

 sometimes by one niorcj and are all annexed to the thorax, or 

 rather cephalo-thorax, which, except in a small number, is 

 only formed of a single segment and is frequently intimately 

 united to the abdomen. This latter part of the body is soft, 

 or but slightly defended, in most of them. 



With respect to their nervous system, the Arachnides are 

 greatly removed from the Crustacea and Insects; for if we ex- 

 cept the Scorpions, which from the knots or joints forming 

 their tail have some additional ganglions, the number of these 

 enlargements of the two nervous cords is never more than 

 'three, and even in the latter, all counted, it never extends 

 beyond seven. 



Most of the Arachnides feed on Insects which they either 

 seize alive, or to which they adhere, abstracting their fluids 

 by suction. Others are parasitical, and live on vertebrated 

 animals. Some of them however are only found in flour, on 

 cheese, and even on various vegetables. Those which live 

 on other animals frequently multiply there to a great extent. 

 Two of the legs, in some species, are only developed by a 

 change of the tegument, and in general it is not until the fourth 

 or fifth change of skin that these animals are capable of pro- 

 pagation(l). 



Division of the Arachnides into orders. 



Some have pulmonary sacs(2), a heart with very distinct 

 vessels, and six or eight simple eyes. They compose our first 

 order, or that of the Pulmonari^. 



The others respire by trachesB, and have no organs of cir- 



(1) We have seen, according' to the observations of Jurine, Juh., that they only 

 acquire this faculty after their sixth change. This fact is also applicable to the 

 Lepidoptera, and probably to other insects that frequently cast their skin, for 

 caterpillars usually change it four times before they enter into the state of a chry- 

 salis which is a fifth. The insect does not become perfect until after another, so 

 that it changes its skin six times. 



(2) Sacs containing air-branchije, or fulfilling the functions of lungs, and distin- 

 guished by me from the latter by the name o{ pneumo-branchiae. 



' Vol. III.— V 



