Body depressed, with pedal filaments, tentacula and branchiæ in two longitudinal rows upon the back.
These Worms all live in the sea, adhering mostly upright in the earth, and have frequently annulate, rigid tentacula, ocelli, and pairs of maxillæ, like Insects. The gills are not unfrequently covered with pergamentaceous scales, as in many Nereides, and especially in Aphrodite.
Fam. 6. Actinioid Worms—Auchenobranchiata.
Body inclosed within a pergamentaceous or calcareous tube with lateral bristles, branchiæ and tentacula upon the neck or head.
The neck is surrounded by a kind of mantle, almost as in the Snails, so that several animals have been arranged here, which are now known to be veritable Snails. Upon the head many have a horny operculum, whereby they can close the shell.
Here belong the Amphitritæ, Terebellæ, Serpulæ, and Sabellæ.
Order 3. Acalephan Worms—Sternwürmer.
3506. These creatures attain the most perfect structure of the Worms. The blood is white, the form cylindrical, globular and stellate; the mouth surrounded by a ring or wreath of maxillæ. The nerves form a ring around the pharynx, and upon the latter are placed membranous cysts, which spirt water into the tentacula or feet as they have been called, and thereby expand them.
It is impossible for these animals to continue to range with the Acalephæ, although they resemble them in outward form; for they consist of two cysts, the intestine having become freed from the outer one as a special sac; moreover, they have a perfect vascular system, distinct muscles, a mouth with a dental apparatus, which prefigurates a complete skeleton, a peculiar vascular system for the injection of the tentacula or feet, a nervous ring surrounding the pharynx, an ovarium entirely separated, and lastly, a perfectly annulate body.
Fam 7. Physalial Worms, Holothuriæ.