The renal organs are still not to be found in all of them; the structure alone of the heart agrees essentially, despite of its simplicity, with that of the Cuttle-fishes. In the Ascidiæ, simple as well as compound species, it is indeed but a single, though muscular pouch. Now, this pouch drives the blood alternately, at one time into the branchiæ, at another backwards into the body, and is thus in the first act a venous or right heart, in the latter an arteriose or left; consequently, according to function this single pouch is compounded or made up of the two hearts.
The branchiæ deviate entirely from those of the Mussels and Snails, and exhibit a very complicated structure; in the mussel-like families they present the form of a trellis-shaped sac, as in the Ascidiæ, or that of filiform appendages to the feet, as in the Cirripedia, or they are funiform as in the Brachiopoda. In the Pteropoda they are very varied; in the snail-like Heteropoda they are mostly pectiniform or tuft-shaped; in the Sepiæ or Cephalopoda phylliform or fin-shaped.
The external character ccommon to the whole class is the cylindrical body, unto which may be aptly conjoined the possession of special organs of motion; whether such organs be prehensile in function, media of support, or veritable instruments of progression.
The snail-like Pteropoda, Heteropoda, and Cephalopoda, have a head, but this is wanting in the mussel-like Salpæ, Ascidiæ, Cirripedia, and Brachiopoda.
THIRD CIRCLE. RESPIRATORY, CUTANEOUS ANIMALS.
3228. Respiratory animals are Dermatozoa with a predominating system of respiration.
Now, the respiratory system is the tegument, which here then must attain the highest pitch of perfection.
This again takes place through the increased process of oxydation, which produces induration of the parts.
The vessels, which surround the tegument, must nourish this portion of it to a greater degree and render it harder or more compact than any other, whereby alternating expansions and contractions thereof originate, in other words, the structure of the trachea.
3229. The whole body of the Respiratory animals becomes a trachea, a series of rings. The Respiratory are therefore the Annulate animals.