_Annelida._—Thus in the worms (_Annelida_), creatures exhibiting but few of the higher manifestations of life, we find the appendages to the trunk in their simplest form,—that of fine, minute bristles attached to the several rings or segments of the body. In the sea-mouse (_Aphrodita_), their bristles attain a somewhat higher developement, each of them being connected with a distinct elevation, or mammillary process, with which it is, as it were, articulated. _Myriapoda._—In the Centipedes, the first evidence of a well-formed extremity makes its appearance in the form of numerous articulated legs terminated by simple points. These legs, rudimentary in the lowest species (_Iulidæ_), in harmony with their retiring habits and vegetable diet, are divided into distinct joints, and provided with proper muscles, in the strong and carnivorous varieties (_Scolopendridæ_). In these latter animals, the ventral nervous ganglia are also increased in size, in a ratio proportioned to the higher and more complex structure of the appendages to the trunk. _Insects._—In insects, creatures distinguished by so many and such admirable instincts, and endowed with such active powers of locomotion, the appendages to the body, both legs and wings, are perfectly and beautifully formed. The former, six in number, are attached to the three first thoracic segments of the body, composed of distinct articulations, connected by ligaments and muscles, and furnished with a series of minute breathing tubes, or tracheal ramifications, extending throughout their entire substance. The latter—the wings—vary much in their number, form, and arrangement; and it is upon characters furnished by them that the nearest approximation to a perfect classification of insects—that of Burmeister—is based. Thus, the legs are typical of the class, the wings characterise the orders. _Arachnida._—In the spider tribe (_Arachnida_) the legs, in their general structure, resemble those of insects; but they are eight in number, and, in addition to them, these creatures are provided with remarkable claw-like appendages to their jaws—mere organs of feeling in the mites, but strong and powerful instruments of prehension in the scorpion tribe. _Crustacea._—The crab and lobster tribe (_Crustaceans_) are likewise accurately characterised by the appendages to the trunk, the number and disposition of these parts having been employed by Milne Edwards for the purpose of dividing these creatures into a number of orders.