198 



ARACHNIDA. 



(Manque de 1'anastomose qui reunit les deux arteres 

 mesenteriques, Mem. de Paris, 1776) ; Du Verney, 

 (Sur les vaissaux omphalo-mesenteriques, Metn.de 

 Paris, 1700) ; Chaussier, (Sur les vaissaux ompha- 

 lo-mesenteriques ; Nouv. Mem. de Dijon, A. 1782. 

 Societ. Philomath, anil); and Tyson,( Unusual con- 

 formation of the emulgents, Philos. Trans. 1678.) 



(J. Hart.) 



ARACHNIDA; aga^i/n, aranea; Eng. 

 arachnidans ; Fr. arachnides ; Germ. Spinne ; 

 Ital. Ragni. 



This class of animals was for a long time 

 confounded with that of insects, but it has been 

 distinguished therefrom by many modem natu- 

 ralists, and more especially by Lamarck, who 

 has applied to it the term l arachnides/ now 

 universally adopted. 



The characters indeed which the arachnidans 

 present are perfectly distinct, and prevent them 

 from being confounded either with crustaceans 

 or insects, although one cannot avoid perceiving 

 that they have numerous relations with the 

 animals of these two classes, and they are con- 

 sequently placed in natural arrangements be- 

 tween the crustaceans and insects. 



Zoologists have assigned the following cha- 

 racters as peculiar to and distinguishing this 

 class. 



Body divided into thorax and abdomen; 

 apterous. Legs, eight in the adult state. Head 

 continuous with the chest. Eyes smooth. Sex- 

 Class. 



ual orifices situated either on the thorax or base 

 of the abdomen. 



To these external characters may be added 

 others derived from the anatomical conditions 

 of different organs. Thus all arachnidans pos- 

 sess exclusively an aerial respiration, either 

 effected by a sort of lungs, or by means of 

 tracheal tubes, as in insects. This difference 

 in the respiratory organs is accompanied with 

 one not less marked in those of circulation ; 

 for example, all the pulmonary arachnidans 

 possess vessels which carry blood, while, on 

 the contrary, all those which have tracheae are 

 deprived of bloodvessels. Lastly, the latter 

 species (or trachearies) alone undergo metamor- 

 phoses analogous, in some respects, to those of 

 insects; while the former (or pulmonaries) 

 suffer only changes of integument. We shall 

 treat further on these peculiarities hereafter. 



Our object here not being to treat of classifi- 

 cation, we shall refer the reader for this subject 

 to the works of Cuvier, Leach, Latreille, 

 Walcknaer, Duges, and limit ourselves at pre- 

 sent to a tabular exposition of the principal 

 divisions and subdivisions admitted in this 

 class down to the genera with which it is most 

 essential to be acquainted. 



Latreille, whose method is that most gene-^ 

 rally adopted by zoologists of every country, 

 divides the arachnidans into two great orders, 

 as follows : 



Orders. 



ARACHNID A / ^ u ^ monar y sacs f r respiration, 6 to 12 ocelli PULMONARIA. 



\ Trachea for respiration, not more than 4 ocelli TRACHEARIA. 



The same author establishes in the first order two families, which are characterized as follows : 



1st Order. 



ARACHNIDA 

 PULMONARIA. 



Palpi simple, pediform; mandibul<e armed with 

 a moveable and perforated claw, emitting a 

 poisonous liquid 



Abdomen inarticulate, terminated by spinnarets 



Palpi produced, cheliform, or shaped like pin- 

 cers 



Mandibula provided with a moveable digit . . 

 ^Abdomen articulate, without spinnarets 



Families. 

 ARANEID&, 



.PEDIPALPI. 



M. Walcknaer, who has made a special of life, has very recently considered them with 



study of the family of araneidte or spinning the express view of arriving at a natural arrange- 



arachnida, and who has published many works ment. of them ; the result of his labour may be 



on their methodical distribution and their habits seen in the following 



