ARACHNIDA. 443 



anatomy and physiology I propose to make the subject of the present 

 lecture, are those commonly known under the name of mites, scor- 

 pions, and spiders. 



You will be disposed to ask why these Articulata are held superior 

 to insects ? They present not only, as in the Crustacea, a more 

 concentrated form of the nervous system and of the heart ; but the 

 larger species, likewise, show a higher condition of the respiratory 

 system, which is less diiFused than in insects, and in some consists 

 only of air-sacs or lungs. A more essential mark of the superiority 

 of the Arachnids is, perhaps, indicated by the course of their develop- 

 ment. The spider undergoes no metamorphoses comparable with 

 those of insects. It is at no period of its development an apodal 

 worm. The mature form is sketched out from the beginning, the 

 divisions of the body characteristic of the perfect animal are 

 established before the vitelline mass is included by the tegument ; 

 and long before the characteristic palpi and legs are completed, the 

 equally characteristic ocelli are developed upon the head. K you 

 should still be disposed to think that the more complex eyes and 

 the wings of insects are essential signs of a higher organisation, 

 by parity of reasoning, you must be prepared to place birds above 

 mammals. The Arachnids, however, form, in fact, a short special 

 branch of the great Articulate tree, beginning very low down, with 

 species hardly higher than the Entozoa or Rotifera, and ascending 

 through the anellid-like Demodex, and the rotifer-like Macrobiotus 

 to the mites, scorpions and spiders. 



The Arachnids, like insects, are organised to live in air ; but they 

 are distinguished at first sight by the general form of the body and 

 the number of their legs, and by some important modifications of 

 their internal structure. The head is always confounded with the 

 thorax, and is deprived of antennae, or at least of such parts ex- 

 clusively employed in sensation ; the homologues of the antennae in 

 insects being metamorphosed into organs of prehension, or weapons 

 of offence. They have four pairs of legs. Some of the species re- 

 spire by pulmonary sacs only, in others these are associated with 

 ramified tracheae, and the smaller Arachnids breathe, like insects, 

 by tracheae exclusively. The dorsal vessel and a circulating system 

 exist in all : the heart presents a more compact and muscular form 

 in the pulmonary Arachnids. 



The integument is chitinous, as in insects, but presents the same 

 variations in density, in different species, as in the winged Articulata. 

 In the scorpions it is as dense and inextensible as in the Coleoptera r 

 in the spiders and mites it is generally softer than in insects, espe- 

 cially that of the abdomen, which is extremely extensible. 



