Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/710

648  only with the observed facts of development. , applying all that was known of logy to the construction of the pedigree of the groups, made (1866) the  one of the five great trunks of the genealogical tree. The  embraced the  and, as well as the , along with the Arthropoda as restricted above. The Arthropoda further formed two groups: &mdash; Carides, the branchiate Arthropods or ; and Tracheata, the, , and , which breathe by. The term disappears from 's latest classification, in which a redistribution of the  is set forth. From assumed ancestors destitute of body-cavity (') descend those Vermes with body-cavity, of whose plan ', Arthropoda, ', and ' show special modifications. In 1870 gave a general table, in which the Vermes included ' and '. The former led towards the '; the latter was the starting point of Arthropoda, ', and ; of the Arthropods there are four es,&mdash;,, , and. Amidst all the varying opinions as to the value of the group, the importance of the limb-structure has been recognised since dwelt upon the articulations by which the parts of these appendages are connected.

The Arthropoda agree in the characteristic already mentioned, the articulations of their limbs, whence the -name is derived. The body presents various degrees of complexity. In the, the ', ', ', or ', owe their mobility to differences in thickness of the integument. In the the numerous similar are flexed on each other by the overlap of the -thickened portions of  which protect the upper and lower surface of each division. The are more or less effaced in the  of  and ;  and  in  and  have their composite origin concealed. But the external signs of division of the body no longer correspond, as in, to the distribution of the internal organs, which, with a partial exception in the case of the , are now unities contributing to the well-being of the whole. Homonomy, the absence of segmentation, or the equivalence of the divisions of the body, among the, has been contrasted with the heteronomy, or segmentation of the arthropod body. The difference, however, is only one of degree, since both the cephalic and caudal extremities, at least of the higher, are true segments, i.e., fused which, in addition to fusion, have undergone some amount of specialisation. In the four es of Arthropods the is a constant segment. It consists of præ- and post-, the of which are represented by the supra- and sub- masses. The number of, as represented by appendages, is not the same in the four es, and as the variation affects the præ- appendages supplied from the supra- the difference is of great importance. In the the  of this segment are, according to ,&mdash;

The Podophthalmata present the s as modifications of processes identical with those which become ambulatory limbs. In the rest the s are sessile. In, , and , the s are sessile, and the præ- appendages are reduced to one pair of , whose innervation is from the supra-. Apart from the value to be assigned to descent in the search for homologies among these es, it is a question of fact whether the s are præ- or lateral to the. The lobes carry the  probably in the earliest types of development at the angle of bifurcation, the position of the single  of. In more complex forms the s appear more or less towards the outer margin of the lobes; and in where the cephalic arch is high, these organs may appear to correspond not to the most anterior, but to a posterior part of the cephalic sterna, just as  appear, the  at the base of the outer, the  at the base of the inner  in the higher. By shortening of the development process the change of position may be obscured, and the s, primitively belonging to the extremity of the nal body, may from the first appear connected with more posterior. The identification of homologous parts of the præ- region in the four es rests on the opinion held as to the origin of the es. If the are regarded as the stock of the Arthropods, the homologies must be recognisable. If, on the other hand, all four are divergencies from a common stock, then the absolute identity of the parts must hold a second place in comparison with a general conformity to the common plan. The identification of the s with a particular pair of appendages necessitates the assumption that these sense organs, when sessile, are so by non-development of their supports. The converse supposition is more admissible, that the s are supported on stalks as the result of an adaptive modification. Further, among the we find hints of the primitive composite character of the Arthropod. The sacs of  are at the caudal extremity of the body, the  of  are in the same position, and the  vary in different, and even in the es of the same. Analogous (perhaps no more) is the distribution among of the  spots which fringe the mantle of Pecten, are pedunculate in the, and, with the otolithic sacs, are in close proximity to the nerve centres of es. If this view is accepted, the close comparison of the limbs of Arthropods loses much of its importance, and it becomes more interesting to endeavour to trace the primitive form from which the divergences have occurred. Among the  is the earliest recognisable form,&mdash;&ldquo;an unsegmented ovate body, a median frontal, and three pairs of natatory , of which the anterior are simple, and the other two biramose&rdquo;. The third pair of appendages is replaced by the, the oval body is divided by a transverse fold, and the  and  thus marked off have the mid-body of the adult developed by intercalation between them. Appendages are developed before segmentation is indicated in the free living ; but in some this stage is overpassed in the, the evidence of its existence being the presence of a thin exuviated which is not  , nor can it be termed amnion, without overstraining that term which is properly used in the higher  logy. In the vermiform stage is rapidly passed through, the priority of segmentation to the development of appendages being indicated in the Trichoptera, according to, and in , according to. If we go to the, there are in that group types which are comparable with the of , and with the vermiform  of , as  and  (Origin and Metamorphoses of Insects) have shown. Pedalion mira has a very close resemblance to the, Lindia to the vermiform  of Dipterous. The resemblance is not impaired by the comparison suggested by between the  and. calls the &ldquo;little more than oligomerous modifications of the polychætous &rdquo; (Nature, December 10, 1874); and in this article it is attempted to show what are the simplest forms presenting common features with the Arthropods. The hexapod has been compared by, , and others to the  of Malacostracous , a group in one