Fig. 1.—Diagram of Arthropod limb extended, retracted, and flexed. Graber has given a similar figure (Insekten, fig. 8*).
Fig. 2.—Vertebrate and Arthropod joints. A, Vertebrate joint, the skeleton clothed with muscles. B, Arthropod joint, the skeleton enclosing the muscles.
In most Arthropoda the body is provided with many appendages. In Crustacea there are often twenty pairs, but some Myriopoda have not far from two hundred pairs. Some of these may be converted to very peculiar functions; in particular, several pairs adjacent to the mouth are usually appropriated to mastication. One or more pairs of appendages are often transformed into antennæ.
The relative position of the chief organs of the body, viz.:—heart, nerve-cord, and alimentary canal, is constant in Arthropoda. The heart is dorsal, the nerve-cord ventral, the alimentary canal intermediate. (See fig. [3].) The œsophagus passes between the connectives of the nerve-cord. Not a few other animals, such as Annelids and Mollusca, exhibit the same arrangement.
Arthropoda are not known to be ciliated in any part of the body, or in any stage of growth. Another histological peculiarity, not quite so universal, is the striation of the muscular fibres throughout the body. In many Invertebrates there are no striated muscles at all, while in Vertebrates only voluntary muscles, as a rule, are striated.
The circulatory organs of Arthropoda vary greatly in plan and degree of complication, but there is never a completely closed circulation.
The development of Arthropoda may be accompanied by striking metamorphosis, e.g., in many marine Crustacea, but, as in other animals, the terrestrial and fluviatile forms usually develop directly. Even in Insects, which appear to contradict this rule flatly, the exception is more apparent than real. The Insect emerges from the egg as a fully formed larva, and so far its development is direct. It is the full-grown larva, however, which corresponds most nearly to the adult Myriopod, while the pupa and imago are stages peculiar to the Insect. It is not by any process of embryonic development, but by a secondary metamorphosis of the adult that the Insect acquires the power of flight necessary for the deposit of eggs in a new site.