MOLLUSCA. 
2G9 
Earth-worms and their allies, which have few bristles on 
each segment (Oligochcetoe ); and the Sea-worms, with nu¬ 
merous bristles, arranged in two clusters on each side of 
each segment ( Polychcetce ). 
These last are the largest of the Worms, and may have 
a distinct head, bearing tentacles and eyes. The oesopha¬ 
gus is often turned in, so as to form a proboscis, which 
bears horny jaws, and can be protruded at the will of the 
animal (Fig. 17). 
Snbkingdom V. —Mollusca. 
A Mollusk is a soft-bodied animal, without internal 
skeleton, and without joints, covered with a moist, sensi¬ 
tive, contractile skin, which, like a mantle, loosely envel¬ 
ops the creature. In some cases the skin is naked, but 
generally 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 class has no distinct 
head. The nervous system consists of three well-devel¬ 
oped pairs of ganglia, which are principally concentrated 
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 aquatic 
animals. Except in the Cephalopods, the muscles are at¬ 
tached to the skin, or shell. There is a heart of two 
chambers (auricle and ventricle) or three (two auricles 
and ventricle). As in all Invertebrates, the heart is arte¬ 
rial. In Mollusks, with rare exceptions, we find no repe¬ 
tition of parts along the antero-posterior axis. They are 
best regarded as Worms of few segments, which are fused 
together and much developed. The total number of 
living species probably exceeds twenty thousand. The 
great majority are water-breathers, and marine; some are 
