# Encyclopædia Britannica, Ninth Edition/Arthropoda

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The Arthropoda agree in the characteristic already mentioned, the articulations of their limbs, whence the class-name is derived. The body presents various degrees of complexity. In the caterpillar, the metamera, somites, somatomes, or annuli, owe their mobility to differences in thickness of the integument. In the Myriapods the numerous similar somites are flexed on each other by the overlap of the chitin-thickened portions of cuticle which protect the upper and lower surface of each division. The somites are more or less effaced in the abdomen of insects and spiders; head and thorax in crabs and spiders have their composite origin concealed. But the external signs of division of the body no longer correspond, as in Annelids, to the distribution of the internal organs, which, with a partial exception in the case of the nervous system, 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 Annelids, 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 Annelids, are true segments, i.e., fused somites which, in addition to fusion, have undergone some amount of specialisation. In the four classes of Arthropods the head is a constant segment. It consists of præ-oral and post-oral somites, the ganglia of which are represented by the supra- and sub-œsophageal masses. The number of somites, as represented by appendages, is not the same in the four classes, and as the variation affects the præ-oral appendages supplied from the supra-œsophageal ganglion the difference is of great importance. In the Crustaceans the somites of this segment are, according to Huxley,—
| App.— | Eyes, | Antennæ, | Antennules, | Mandibles, | Maxillæ, | Maxillæ. |
| Som.— | 1. | 2. | 3. | 4. | 5. | 6. |
Natural as may seem the assemblage included under Arthropoda, there is no group in which adaptive modifications have introduced so much diversity of anatomical and physiological relations. Metamorphoses, the changes of form which changes of external conditions have promoted, are met with of very various amount. The progress of the embryo from the first appearance of the blastoderm up to sexual maturity of the adult may be direct, without metamorphosis, or may be retarded by changes of form and habit, rendering the young animal capable of sustaining life under very various conditions. In any one of these stages, even in the adult, multiplication may be provided for by a process of budding, the bud from which the new form emerges being in essence undistinguishable from the ovum for whose further development impregnation is necessary. These metamorphoses are probably of late origin in the history of the group, their perpetuation being due to change in their surroundings. Their relations may be “falsified by the struggle for existence,” the details of the developmental history of the family (phylogenesis) may be crowded into a short space in the development of the individual (Ontogenesis). The description of these variations belongs to the particular treatment of the Crustaceans, Myriapods, Arachnids, and Insects.
(j. y.)