(J CRUSTACEA. 



besides other characters, distinguish Crustacea from Insecta. A close 

 approximation of Crustacea to other Articulata is not to be expected, 

 except with aquatic species. And we find an analogy with the 

 aquatic larvae of many insects, and still closer with certain Vermes, as 

 the Helminths, into which the lower grades, as they are more and. 

 more nude of members, evidently pass. A strongly-drawn line is 

 here not to be expected, since both are divisions of the same sub- 

 kingdom, and both are fitted for the same element and similar modes 

 of life. 



The position of Crustacea in the scale of animals among the Articu- 

 lata has been learnedly discussed by Professor Agassiz ;* and he has 

 shown, we think satisfactorily, that they should rank below Insects, 

 and above Worms. The gradation into Worms is evidence on the 

 latter point, and the analogy to the aquatic larves of Insects on the 

 former. This author remarks upon the greater number of limbs in 

 Crustacea, which is also a peculiarity commonly of the imperfect insect 

 or larve, the smaller number in the mature insect being a result of 

 a higher stage of development. He also observes that the separation 

 of the head and thorax is further indication that Insects rank highest. 

 It is true that Crustacea attain a size never found among the Insecta. 

 But this is attributable to their living in a denser element, and is 

 analogous to the occurrence of the largest but lowest of Mammalia 

 in the ocean. Moreover, size of body is no necessary criterion of 

 relative rank, for, unless the nervous system is of a higher grade as 

 the size increases, the bulk is so far only an encumbrance to the weak 

 forces within, and less agility of motion and inferior attributes in 

 other respects are the consequence. Thus the huge medusa is but an 

 unwieldy mass compared with minute acalephs, and the large crab but 

 a clumsy animal alongside of the nimble ant. 



It seems to be a correct principle laid down by Professor Agassiz, 

 that in each zoological group the aquatic species are inferior in grade 

 to those of the land. 



II. SUBDIVISIONS OF CRUSTACEA. 



In the classification of Crustacea, we adopt mainly the grand divi- 

 sions laid down in the excellent treatise by Milne Edwards, although 



* Classification of Insects from Embryological Data. By Prof. Agassiz. 28 pp. 4to., 

 and one plate. Smithsonian Contrib. to Knowledge, vol. ii., art. 6. 



