3177. Did the former animals remain in the condition of ova, from want of a perfect vascular system; they are next developed, so soon as vessels appear and form a vascular plexus, into fœtal involucra or envelopes. These animals are vitellus with the vascular membrane.
3178. The Absorbent animals are no longer simple vesicles, but large cysts or capsules like the developmental envelopes of the fœtus, along with a choroid plexus—Involucral, Fœtal-animals.
This choroid plexus does not, however, consist of arteries and veins; but is only a ramification of the intestine, so that the vessels are of a lacteal character—Absorbent animals.
3179. In these animals there is no longer any egg-shell, but everything has been taken up into the galvanic circle; the shell has itself become organic and life-imbued. Their substance is still mucous or albuminous; they are still vitellus, though converted into a vascular tissue.
3180. They therefore cling firmly nowhere; but swim about freely, like brain-masses converted into radiated cysts.
3181. Free Mucus-animals, traversed by vascular plexuses, are Acalephæ.
3182. There are Acalephæ which are simply air-sacs, like the air-sacs of ova, to which hang ramified vessels as absorbent tubes—Cystic, Tubular Acalephæ.
Others represent hemispheres with numerous absorbent tubes, which concur in the middle of the animal to form a kind of stomach, from which again other tubes pass towards the border, in order to elongate into tentacula. Thus the absorbent vessels have become motor and sensitive organs.
Besides this, many have around the mouth four large lobes, which must be viewed as the antetypes of the sentient lobes of the Bivalve Mollusca.
Lastly, others have a true mouth, which leads to a similar gastric cavity, out of which the same vessels emerge and ramify. Both kinds are called Hutquallen or Acalephæ.