/. D. Dana on the Classification of Crustacea. 29 



2. A diminution of force as compared with the size of the 

 structure, leading to an abbreviation or obsolescence of some 

 circumferential organs, as the posterior thoracic legs or anterior 

 antennas, or the abdominal appendages (where such appendages 

 exist in the secondary type embracing the species). These cir- 

 cumstances, moreover, are independent of a degradation of in- 

 telligence, by an extension of the sphere of growth beyond the 

 proper limits of the sphere of activity. 



VII. A classification by grades, analogous to that deduced for 

 Crustacea, ma}^ no doubt be made out for other classes of animals. 

 But the particular facts in the class under consideration, are not 

 to be forced upon other classes. Thus, while inferiority among 

 Crustacea is connected with a diminished number of annuli ce- 

 phalically absorbed (for the senses and mouth), it by no means 

 follows, that the Insecta, which agree in the number of cephalic 

 annuli with the lower Crustacea, are allied to them in rank, or 

 inferior to the higher species. On the contrary, as the Insecta 

 pertain to a distinct division, being aerial instead of aqueous 

 animals, they can be studied and judged of, only on principles 

 deduced from comparison among insects themselves. They are 

 not subject to Crustacean laws, although they must exemplify- 

 beyond doubt, the fundamental idea at the basis of those laws. 



The views which have been explained, lead us to a modifica- 

 tion, in some points, of the classification of Crustacea. The 

 question, whether the eyes are pedicellate or not, upon which the 

 names Podophthalmia and Edriophthalmia are based, proves to 

 be one of secondary importance. And although still available 

 in distinguishing almost infallibly the species of the first type, it 

 is far from rendering it necessary or natural to embrace to- 

 gether under a common division the species that have sessile 

 eyes (so-called Edriophthalmia), as done by most writers on this 

 subject. 



The term Decapoda, in view of these principles, has a higher 

 signification than has been suspected since by expressing the 

 number of feet, it implies the number of cephalic annuli charac- 

 terizing the species. It would not be employing it inconven- 

 iently, therefore, if it were extended to embrace all the Podoph- 

 thalmia, or all species of the first type, including the My sis and 

 Squilla groups. 



For a like reason, the term Tetradecapoda has a high signifi- 

 cance, as applied to the species of the second type. The position 

 of the Trilobita still remains in doubt. The Cirripedia and 

 Entomostraca, third and fourth types, stand properly on nearly 

 the same level. 



