304 CRUSTACEA. 



that we should place these singular fossil animals, originally con- 

 founded under the common name of Enlomolithus paradoxvs, and non- 

 designated by that of Trilobites. By this hypothesis we have to 

 admit as a positive, or at least highly probable fact, the existence 

 of locomotive organs, although, notwithstanding the most careful 

 investigation, no vestige of them has been discovered. Presuming, 

 on the contrary, that these animals were deprived of them, I thought 

 that their natural position was in the neighbourhood of the Chitones, 

 or rather that they constituted the original stock of the Articulata, 

 being connected on the one hand with these latter Mollusca, and 

 on the other, with those first mentioned, and even with the Glomeres, 

 to which some Trilobites, such as the Calymenes, appear to ap- 

 proximate, as well as to the Chitones, inasmuch as by contracting they 

 could also become spherical. Be this as it may, these animals appear 

 to have been annihilated by some ancient revolution of our planet. 



The Trilobites, one heteromorphous genus excepted, that of Agnostus, 

 have, like the Limuli, a large anterior segment in the form of an almost 

 semicircular or lunated shield, followed by from about twelve to twenty- 

 two segments, all transversal except the last, and divided by two longi- 

 tudinal sulci into three ranges of parts or lobes, whence their name of 

 Trilobites. Some naturalists call them Entomostracites. 



They are divided by M. Brongniart into the following genera : 

 Agnostus, Calymene, Asaphus, Ogygia and Paradoxides. 



CLASS II. 



ARACHNIDES. 



THE Arachnides, which compose the second class of articulated 

 animals provided with moveable feet, are, as well as the Crustacea, 

 deprived of wings, are not subject to changes of form, or do not expe- 

 rience any metamorphosis, simply casting their skin ; but they differ 

 from them as well as from insects in several particulars. Like the 

 latter, the surface of their body presents apertures or transverse 

 fissures called stigmata, for the introduction of air, but they are few in 

 number eight at most, and usually but two and confined to the 

 inferior portion of the abdomen. Respiration is also effected either by 

 means of air-branchiae, fulfilling the functions of lungs that are con- 

 tained in sacs, of which these stigmata are the apertures, or by radiated 



