CONSIDERATIONS. 1 !37 



these organs in the Annelida, we find their situation gradually 

 becoming lower down in the alimentary canal, to acquire that 

 disposition they present in Crustacea and Insects. In Nephtys 

 and other genera, the jaws are placed at the lower part of the 

 oesophagus, which reverses itself in the form of a proboscis 

 when the animal wishes to feed. In comparing the jaws of 

 the Annelida to the organs of the mouth and stomach of the 

 Annulosa, we find them, both as to form and disposition, more 

 analogous to the gastric than the oral jaws of the latter. 



The transformation of the first pair of feet into maxillae is 

 very evident in the Scolopendrce, but it is not so with respect 

 to the labium and mandibles : perhaps there exists some spe- 

 cies as yet unknown, which may afford us a proof of a similar 

 change in these also. 



The Crustacea have from two to six pairs of jaws, the 

 posterior pairs in many closely resembling the feet, proving 

 indubitably that the organs of the mouth are but these last 

 modified. The strength of the mandibles, and the size and 

 number of the maxillae, show that these animals subsist on 

 solid food; but the nature of this is not always clearly marked 

 by the form of the jaws, though in general the carnivorous 

 species have them toothed, the herbivorous merely incisive. 

 In the parasitical Crustacea {Nynrphon, Phoxichilus, (yc.) 

 the mouth, though formed on the same plan as that of the 

 other Crustacea, is smaller and much less developed; hence 

 these animals subsist by sucking the blood of other animals 

 (generally the Cetacea) instead of solid food. All the Arach- 

 nida {Arachnida and Acaridea) are very rapacious, but the 

 parts of the mouth offer a striking difference in form, the 

 larger species (the Arachnida , MacLeay), which prey on 

 insects, having them very robust, but suited more to their 

 habits of sucking their prey than to the purposes of mandu- 

 cation, whilst in the smaller {Acaridea, MacLeay), which are 

 mostly parasitical, they are commonly formed into a simple 

 haustellum. Some however of these, as the Acari, which 

 feed on dry animal and vegetable substances, are masticators. 



The development of the mouth attains its greatest degree 

 of perfection in the Coleoptera, and we can, with some few 

 exceptions, determine the nature of their food from the form 

 of the tropin. Those which prey on living animals have 

 the mandibles slender, and projecting beyond the labrum about 



