3137. In the branchiæ, however, it is only the cauline portions which become horny, while their ramules continue to perform the respiratory function. The branchiæ therefore are appended to the extremity or roots of the feet; or rather these latter form the branchial arches.
Annulate animals, having true or hornified and likewise annulated feet, are called Crustacea or Crabs.
Thus, in these animals the sense of feeling obtains special and moveable organs; they are Tactile animals.
Tactile organs are prolongations of tegument moved by muscular fibres, which in some degree adapt themselves by pressure unto the forms of objects, or have the power of seizing and retaining them—such as feet, antennæ, maxillæ, palpi.
3138. But if the tegument be entirely converted into horn, and the respiratory vessels have in this way disappeared within it, then the internal respiratory organs are formed by inversion of that part of the tegument which is between the rings, and through these openings the air penetrates to the internal parts—stigmata, tracheæ.
3139. The tracheæ can first of all originate when the respiratory process has attained its highest development, or, in other words, when the creature breathes air.
3140. Lastly, in the air-breathing Ancyliozoa even the external branchial lamellæ harden and are converted into wings—Pterozoa, Insects or Flies.
The wings of Insects do not correspond to the wings of birds; they are not feet, but pedal appendages or branchiæ, and thus are no new or unknown organ.
Circle IV. Sarcose Animals.
3141. The second animal province may be regarded as the fourth stage in the self-substantial development of the anatomical systems, although, properly speaking, it ranks as regards its value or worth upon a level with the three former circles, and resolves itself directly into three stages, which accord with its three systems. But since these stages are at the same time also classes; they should, for uniformity's sake, retain the latter name.