The rings are to be regarded as cysts, which are mutually juxtaposited or repeated, not being inserted within each other as in the preceding animals, but behind each other, so that the present circle consists of multiplied Cystic animals.
3230. The Annulate animals must represent the developmental stages of the tegument, while the viscera make a retrograde step—Tactile animals.
3231. The tegument has two functions: it is an organ of respiration and of general feeling or sensation. In both cases it passes through three stages of development.
It is either entirely a branchia, i. e. reticular in character; or the branchiæ become individualized in certain situations and partly horny, constituting branchiæ proper; or finally, the tegument is converted into an air-breathing organ, tracheæ or air-tubes.
3232. It either feels with its whole surface, or by papillæ; or the branchiæ change into horny feet; or lastly, into alary appendages or wings. The Ancyliozoa consequently divide into Reticular or Papillary animals; into Branchial or Pedal, and into Tracheal or Winged animals.
3233. As the arterial character preponderates in these animals, so do the venous organs, especially the liver and renal structures, retrograde.
3234. The body is now an intestinal and tegumental body with predominating respiratory and sentient organs; here therefore sensitive papillæ, antennæ, feet and wings, appear in abundance.
3235. With the retrogression of the viscera, the glandular structure also and large proportional bulk of the sexual parts disappears. They assume the form of the intestine and tegument, i. e. become tubes.
The spawn like the milt consists, as a general rule, of only two long membranous tubes, running near the intestine.
3236. The external sexual parts belong to the sense of feeling and follow the developments of the tegument; they make their appearance as antennæ, and are usually accompanied by foot-like accessory organs.