THIRD SUB-KINGDOM. 



MOLLUSC A. 



This sub-kingdom ranks side by side with the preceding. The 

 highest Molluscs are superior to the highest Articulates ; while in both, 

 the lowest are inferior to many Radiates. In Articulates, there is an 

 advance manifested in the powers of locomotion and in the instincts ; 

 but in the Molluscs, the energies are expended chiefly in the perfection 

 of those organs concerned in the immediate preservation of the individ- 

 ual and the species. 



Molluscs have the centre of the nervous system scattered unsymmet- 

 rically (whence Owen's name of the Sub-kingdom — Heterogangliata) ; 

 and this is often accompanied with an unsymmetrical form of the entire 

 body. All have a complete alimentary canal, with mouth, stomach, in- 

 testine and vent ; a liver, but no absorbent system ; a chambered heart 

 propelling a colorless blood through a system of arteries and veins ; and 

 either lungs or gills. They are hermaphrodite or bisexual, oviparous or 

 viviparous. Two-thirds of the species are aquatic ; many are exclusively 

 oceanic, some are restricted to the brackish water of estuaries ; others 

 live only in fresh water ; and some on land. Every region has its ap- 

 propriate tribe j and so has each zone of depth in the sea, from the 

 floating Pteropods to the Terebratuke sometimes found at 100 fathoms. 

 Deep water shells are mostly small, and have a wide range and high 

 antiquity. As a general rule, the land Molluscs are biennial ; the aquatic 

 species, annual. In compensation for the low condition of the perceptive 

 energies, the body of the Mollusc is commonly protected by one or more 

 plates or shells, variously adjusted upon the soft parts of the animal. 



Three-fourths of living species of Molluscs are univalves; the others are 

 mostly bivalves. The bivalves are all aquatic. With rare exceptions, 

 land, fresh water and littoral univalves have entire mouths and are 



