3206. The organization of the Bivalve Mollusc can be thus most distinctly described; it is an abdomen, wherein is an intestine with mouth and anus, a liver and a double ovarium; upon the sides of this abdomen are situated the branchiæ in the form of four laminæ; around the branchiæ and the abdomen is the pleura or mantle, which is always open posteriorly.
3207. The mouth is placed directly at the anterior extremity of the abdomen, is devoid of neck and head, as also salivary glands; it is consequently not a true mouth, but only a pharyngeal aperture. Thereupon, however, are situated four sensitive lobes, which are in structure exactly like the branchiæ—cephalic or pharyngeal branchiæ. They are the further formations of the four arms of the Acalephæ.
3208. The Mussel has a perfect splanchno-neural system with ganglia and a pharyngeal ring, which probably corresponds to the nerves that sweep around it.
3209. The Mussel has no other organ of sense than that of the passive sense of feeling, the tegument. It cannot once move its sensitive lobes voluntarily; it has no lips.
3210. The abdomen only elongates in most of them to form a moveable, variously constructed foot or keel, which cannot, however, creep but only push. The progression of the Bivalve Mollusca is backwards, as in the Acalephæ. In the Snails the ventral surface first becomes a creeping sole.
3211. The Mussels repeat the Infusoria; are Infusoria with a bivalve calcareous testa or shell.
Class 5. Arterial, Orchitic Animals.
3212. In the preceding class of Bivalve Mollusca, it is in truth the abdominal viscera only, such as the intestine, liver and ovarium, which have been perfected; and then the veins and arteries with a membranous heart. The cephalic organs, eyes, maxillæ, salivary glands, and even moveable lips with tentacula, are wanting, as well as the muscular heart. Lastly, the arterial sexual system or a self-substantial testis and the penis were absent.
Now, Mollusca, which have eyes, maxillæ, a muscular heart and a ventral sole or foot, with salivary glands and a penis, are Gasteropoda or Snails.
3213. The Snails possess salivary glands, a trace of the tongue and of maxillæ, moveable lips and tentacula, with thus an approximation to the head, unto which the eyes are rarely wanting—Salivary animals.