ZOOLOGY. 523 



mouth, but only a pharyngeal aperture. Thereupon, 

 however, are situated four sensitive lobes, which are in 

 structure exactly like the branchiae cephalic or pharyn- 

 geal branchiae. They are the further formations of the 

 four arms of the Medusae. 



3208. The Mussel has a perfect splanchno-neural sys- 

 tem with ganglia and a pharyngeal ring, which pro- 

 bably 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 pro- 

 gression of the Bivalve Mollusca is backwards, as in the 

 Acalephae. In the Snails the ventral surface first becomes 

 a creeping sole. 



3211. The Mussels repeat the Infusoria; are In- 

 fusoria 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, maxillae, salivary 

 glands, and even moveable lips with tentacula, are want- 

 ing, 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, maxillae, 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 maxillae, moveable lips and tentacula, with 

 thus an approximation to the head, unto which the eyes 

 are rarely wanting Salivary animals. 



3214. With the development of the head or rather of 



