ARACHNIDES. 277 



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 propagation* 



Division of the Arachnides into Orders. 

 Some have pulmonary sacsf, a heart with very distinct vessels, 

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



of the PuLMONARIiE, 



The others resjjire by tracheae, and have no organs of circulation, 

 or, if they have, the circulation is not complete. The tracheae are 

 divided near their origin into various branches, and do not, as in 

 Insects, form two trunks which run parallel to each other throughout 

 the whole length of the body, and receive air from various points by 

 means of numerous stigmata. Here, but two, at most, are distinctly 

 visible, and they are situated near the base of the abdomen J. The 

 number of simple eyes is at most but four. They constitute our 

 second and last order, or that of the Track earle. 



ORDER I. 



PULMONARIiE§. 



We here find a well marked circulating system and pulmonary sacs, 

 always placed under the abdomen, announced externally by transverse 

 openings or fissures (stigmata), of which there are sometimes eight, 

 four on each side, and at others four, or even two. The number of 

 simple eyes is from six to eight|[, while in the following order it 



* We have seen, according to the observations of Jurine, Jun., that they only 

 acquire this faculty after the 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. 



t Sacs containing air-branchiae, or fulfilling the functions of lungs, and distin- 

 guished by me from the latter by the name of pneumo-brancliue. 



X The Pycnogonides exliibit no stigmata, and seem, in this respect, to approach 

 the last of the Crustacea, such as Dichelestium, Cecrops, and other Siphonostomous 

 Entomostraca. Savigny thinks they have a closer affinity to the Lremodipoda, 

 from which, however, they are greatly removed, by the organization of the mouth as 

 well as by their eyes and feet. We still believe, however, from the ensemble of their 

 characters, that they rather belong to the class of Arachnides, and that they approxi- 

 mate particularly to Phalangium, with which various authors have arranged them. 

 We also think that they may respire by the surface of theii- skin. At all events, we 

 must await the results of anatomical investigation before we can decide. 



§ Unogata, Fab. 



II The Tessarops of Rafin, according to him, has but four eyes ; I presume, liow- 

 ever, that the lateral ones escaped his notice. Sec the subgenus. £<c5«s. 



