ARACHNIDES. 
277 
multiply there to a great extent, Two of the legs, in some species, 
are only developed by a cliange 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 respire 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 re'ceive 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 TraciiearijE. 
ORDER I. 
PULMONARI^g. 
We here find a well marked circulating system and pidmonary sacs, 
always placed under the abdomen, announced externally by transverse 
openings or fissures (stigmata), of which there ai’e 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 obsei'vations of Jurine, Jun., that they only 
acquire this faculty after the sixth change. This fact is also applicable to the 
Lcpidoptera, 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. 
f Sacs containing air-branchiae, or fulfilling the functions of lungs, and distin- 
guished by me from the latter by the name of pneumo-bmnchiee. 
J The Pycnogonides exhibit 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 Laeinodipoda, 
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 their skin. At all events, we 
must await the results of anatomical investigation before we can decide. 
§ Unog.\ta, Fab. 
II The Tessarops of Rafin, according to him, has but four eyes ; I presume, how- 
ever, that the lateral ones escaped his notice. See the subgenus Eresus. 
