Cylindrical, with a tubuliform mantle almost entirely closed and frequently surrounded by a calcareous tube, in addition to the two ordinary shell-valves; the pedal aperture is situated at the oral extremity; the respiratory tubes retractile by means of muscles; the foot is cylindriform.
Of the same kind are the Pfahlmuscheln, Teredines, and Sandmuscheln, as also the Solenidæ. They typify the Nudibranchiate Snails and the Salpæ.
Fam. 2. Polypary Mussels—Klaffmuscheln or Tellinidæ.
Body flat, mostly discoidal; there is a large fissure on the abdominal side of the mantle for the passage of the foot, while posteriorly there are two long respiratory tubes with retractor muscles. Here belong the Tellinidæ and Venusidæ.
I have shown, that they are already to be recognized by the uncinate or hook-shaped groove for the mantle in the shell, and which proceeds from the insertion of the retractor muscles. The foot is usually lancet-shaped. They typify the Patellæ and Ascidiæ.
Fam. 3. Acalephan Mussels—Cardiacea.
Body globular in form, mantle having a pedal slit on the abdominal side; two respiratory apertures without retractor muscles; foot mostly uncinate or strap-shaped.
T have shown that these creatures are to be recognized by a discoid groove in the shell, which simply depends upon the mantle, the retractor muscles being wanting. Here belong the Cardiacea and gigantic Chamidæ, in which last a byssus occurs, and the union also of the two occlusor or sphincter muscles, as in the following order. They antetypify the air-breathing Snails and Cirripedia.
Order 2. Conchozooid Mussels—Spaltmuscheln.