382 ON THE ORDERS 



ceed and communicate with every respiratory cavity, ra- 

 mifying over its membrane. When the stigmata, on the 

 contrary, are tracheal, there is no circulation •, but in its 

 place we have alternate contractions of a dorsal vessel, 

 which, fiom its not throwing off any branch whatever, 

 appears to deserve the name of heart no further than as it 

 occupies a situation in these animals, which corresponds 

 to that of the heart in Pulmonary Arachnida. Neverthe- 

 less the Arachnida, whether breathing by pulmonary 

 pouches or by tracheae, form but one class, as we perceive 

 from the union of their head and thorax into one piece, 

 and from the concentration of their viscera into the abdo- 

 minal portion of the body. It is needless to repeat in this 

 place the arguments that have been already adduced to 

 show, that the division of the organs of respiration and 

 circulation is not to be depended on in the classical ar- 

 rangement of the Annulosa. If these arguments should 

 not have been deemed satisfactory, perhaps the opinion of 

 M. Lamarck on the subject may influence the reader to 

 believe, that their only weakness has consisted in my man- 

 ner of enforcing them. "Lorsqu'il y a de grandes analo- 

 gies d'ensemble, les diverses particularitts d' organization 

 que Von observe quelquefois ne permettent cependant pas 

 de separer classiquement les objets qui les offrent. Qu'i/ 

 a-t-il, en effet, dephisvoisin des araignees que lesfaucheurs, 

 les galeodes, S;c.! Cependant les premieres respirent par 

 des poches tvidemment branchiales, tandis les autres ne 

 respirent que par des trachees" The tracheae, however, 

 of the Arachnida, when they exist, differ from those of 

 Insects in being disposed in a solitary ramifying series, 

 which may almost be termed radiated ; on the other hand, 



