210 L. Agassiz on the Natural Relations between 



tacea, the natural circumscription of which can hardly be in 

 any degree a matter of doubt, for these animals, with their 

 distinct articulations, and aquatic mode of respiration, exter- 

 nal appendages and particular mode of combination of the 

 rings of their body, wherever they are combined to subdivide 

 the body into distinct regions, are so peculiar as to determine 

 well the natural limits of this class, to which we refer also 

 the Cirripeda, notwithstanding their transformations, also 

 the Lernsean parasites, though they may assume in their para- 

 sitic mode of existence so extravagant forms, and an appear- 

 ance so entirely different from that of common Crustacea. 

 In this class, again, the parasitic vermiform types rank lowest; 

 next follow the Entomostraca, and highest the Malacostraca, 

 in most of which the anterior rings are combined into a dis- 

 tinct region, assuming a peculiar appearance, differing widely 

 from the posterior free movable rings. The circumstance 

 that among Crustacea the organization reaches a point where 

 the anterior part of the body assumes so peculiar an appear- 

 ance, leaves no doubt as to the relative position of Crustacea 

 among Articulata ; they rank higher than worms ; though 

 they must be placed below the insects, notwithstanding their 

 perfect circulation, and their otherwise highly developed 

 structure ; for, in every respect, insects considered as a whole 

 class, are more highly organized, their higher types assuming 

 a division of the body into three distinct regions, — undergo- 

 ing also far more extensive metamorphosis, and assuming, 

 finally, an aerial mode of respiration, to which the Crustacea 

 do not reach. For these reasons, which I have illustrated 

 more fully on another occasion, I have no hesitation in placing 

 the class of Insects highest among Articulata, and in compris- 

 ing in one class the true insects with Arachnida and Myrio- 

 poda, which are only lower degrees of development of the 

 more special types of true insects ; the Myriopoda represent- 

 ing, in a permanent state of development, and with the 

 structure of true insects, the form of their caterpillars ; the 

 spiders, with their cephalic and thoracic regions united into a 

 cephalo-thorax, representing their chrysalis in a permanent 

 state of development ; and the true insects, with their three 

 distinct regions, the so-called head, thorax, and abdomen, 



