254 COMPARATIVE ZOOLOGY. 
especially in tropical waters, where they sometimes attain 
the length of three or four feet. As found on the beach 
after a storm, or when the tide is out, they are leathery 
lumps, of a reddish, brownish, or yellowish color. They 
may be likened to a Sea-urchin devoid of a shell, and 
long drawn out, with the axis horizontal, instead of ver- 
tical. 
Subkingdom Mot.vsca. 
A Mollusk is a soft-bodied animal, without internal 
skeleton, and without joints, covered with a moist, sensi- 
tive, contractile skin, which loosely envelops the creature, 
like a mantle. In some cases the skin is naked; but gen- 
erally it is protected by a calcareous covering (shell). The 
length of the body is less in proportion to its bulk than in 
other animals. The lower classes have no distinct head. 
The nervous system consists, in the true Mollusks, of three 
well-developed pairs of ganglia, which are principally con- 
centrated around the entrance to the alimentary canal, 
forming a ring around the throat. The other ganglia are, 
in most cases, scattered irregularly through the body, and 
in such the body is unsymmetrical. The digestive system 
is greatly developed, especially the liver, as in most aquat- 
ic animals. Except in the Cephalopods, the muscles are 
attached to the skin. Only the higher Mollusks have a 
distinct heart (auricle and ventricle), and this is always on 
the arterial side. While in neighboring groups, as Fora- 
minifers, Corals, Star-fishes, and Articulates, we find repe- 
titions of similar parts, in Mollusks every part or organ is 
single. The total number of living species probably ex- 
ceeds 20,000. The great majority are water - breathers, 
and marine; some are fluviatile or lacustrine; and a few 
are terrestrial air-breathers. All bivalves, and nearly all 
univalves, are aquatic Each zone of depth in the sea has 
its particular species. 
