MOLLUSCA. 



269 



Earth-worms and their allies, which have few bristles on 

 each segment (Oligochcetce) ; 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). 



Subkingdom 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 



