COMPOSITION OF THE INSECT-CRUST. 9 



brought as if towards a focus, and that focus the head, which 

 is the meaning of the term " cephalization," proposed by Pro- 

 fessor Dana.* Ring distinctioiis liave given way to regional 

 distinctions. The former characterize tlie Worm, the latter 

 the Insect. In other words, tlie division of tlie body into three 

 parts, or regions, is in tlie insect, on the whole, better marked 

 than the division of any one of those parts, except the abdo- 

 men, into rings. 



Composition of the Insect-crust. Before describing the 

 composition of the body-wall, or crust, of the Insect, let us 

 briefly review the mode in which the same parts are formed in 

 the lower classes, the Worms and Crustaceans. We have seen 

 that the typical ring, or segment (called by authors zoonule, 

 zoonite, or somite, meaning parts of a body, though we prefer 

 the term arthromere, denoting the elemental part of a jointed 

 or articulate animal), consists of an upper (tergite), a side 

 (pleurite) , and an under piece (sternite) . This is seen in its 

 greatest simplicity in the Worm (Fig. 2), where the upper and 

 ventral arcs are separated by the pleural region. In the Crus- 

 tacean the parts, hardened by the deposition of chitine and 

 therefore thick and unyielding, have to be farther subdivided to 

 secure the necessary amount of freedom of motion to the body 

 and legs. The upper arc not only covers the back of the ani- 

 mal, but extends down the sides ; the legs are jointed to the 

 epimera, or flanks, on the lower arc ; the episternuin is situated 

 between the epimerum and sternum ; and the sternum, form- 

 ing the breast, is situated between the legs. In the adult, there- 

 fore, each elemental ring is composed of six pieces. It 

 should, however, be borne in mind that the tergum and ster- 



* In two papers on the Classification of Animals, published in the American 

 Journal of Science and Arts, Second Series, vol. xxxv, p. 65, vol. xxxvi, July, 1863, 

 and also in his earlier paper on Crustaceans, "the principle of cephalization is 

 shown to be exhibited among animals in the following ways : 



1. By a transfer of members from the locomotive to the cephalic series. 



2. By the anterior of the locomotive organs participating to some extent in ce- 

 phalic functions. 



3. By increased abbreviation, concentration, compactness, and perfection of 

 structure, in the parts and organs of the anterior portion of the body. 



4. By increased abbreviation, condensation, and perfection of structure in the 

 posterior, or gastric and caudal portion of the body. 



5. By an upward rise in the cephalic end of the nervous system. This rise 

 reaches its extreme limit in Man." 



