1 92 THE SCIENCE OF LIFE. 



CHAPTER XIII. 



MOLLUSCA. 



I have seen 



A curious child applying to his ear 

 The convolutions of a smooth-lipped shell, 

 To which, in silence hushed, his very soul 

 Listened intensely, and his countenance soon 

 Brightened with joy ; for murmuring from within 

 Were heard sonorous cadences whereby, 

 To las belief, the monitor expressed 

 Mysterious union with its native sea. 



WORDSWORTH. 



1. THE type of Mollusca, or soft-bodied animals, is in- 

 dicated by the name, derjved from the Latin mollis, soft. 

 Like other types it embraces species of various degrees 

 of complexity of structure, and of various forms. It in- 

 cludes soft-bodied, unjointed animals, possessing a mus- 

 cular skin, or mantle, generally protected by a calcareous 

 shell, and whose nervous system is scattered. It is sub- 

 divided into i. MOLLUSCOIDA, containing the classes 

 Polyzoa, Tunicata, and Brachiopoda; and 2. TRUE MOL- 

 LUSCA, embracing the classes Lamellibranchiala, Gaster- 

 opoda, and CepJialopoda. 



2. POLYZOA (Gr. polus, many, and zoon, animal) derive 

 their name from the fact of their living in clusters or 

 colonies. They are sometimes called Byozoa, (Gr. byon, 

 moss, and 20011, animal,) or Sea-moss. They greatly re- 

 semble the Hydroid polyps, but from the greater complex- 

 ity and character of their organization they have been 



