SUBKINGDOM 



MOLLUSCA. 



Tins subkiugdoni ranks side by side with the articiilata, and 

 the lowest forms of the two subkingdoins have snch relationship- 

 that by some authors they are grouped as a distinct subkingdom. 

 Linnaeus placed all the mollusks with the vermes, and they are^ 

 perhaps, a specialized group of the latter. The highest mollusks 

 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 intelligence ; in 

 the mollusks the energies are expended chiefly in the functions 

 of nutrition and reproduction. 



Mollusks have the centres of the nervous sj'stem scattered un- 

 symmetrically, and this is often accompanied with an unsym- 

 metrical form of the unsegmented body. They have a complete 

 alimentary canal, with mouth, stomach, intestine and vent; a 

 liver, but no absorbent system ; a chambered heart propeling 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 appropriated tribe ; and so has each zone of 

 depth in the sea, from the floating Pteropods to the Terebratulse^ 

 sometimes found at 100 fathoms. Deep water shells are mostly 

 small, and have a wide range and high antiquity. In compensa- 

 tion for the low condition of the perceptive energies, the body 

 of the mollusk is commonly protected by one or more plates or 

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



Living species of molluscs number about 20,000, of which 

 three-fourths are univalves; the others are mostly bivalves. The 

 bivalves are all aquatic. 



Mollusks are separable into the cephalous and acephalous. 

 The former are nearly all univalves, and are subdivided according 



