SINGLE-SHELLED MOLLUSKS. 61 
(Fig. 63, 4). They also have an odontophore or tooth- 
bearer, and the shell is often closed by a plate or door 
called the operculum ; 22,000 species are known, 7,000 
of which are extinct or fossil. 
Sea Wood-Lice (Chitonide).—In these (Fig. 64) the 
shell is composed of eight transverse plates. The young 
Fic. 64.—A gasteropod (Chzfon). 1. Adult, showing plates. II. Chzton 
dissected: 0, mouth; g, nervous ring; ao, great artery from the heart, 
aorta; ¢, ventricle; c’, an auricle; dr, left branchix ; od, oviducts. III, 
IV, V. Development of free-swimming young. 
(Fig. 64, III) at first have no shells, swimming about by 
the aid of minute cilia; 250 living species are known, 
and 125 fossil. In the West Indies they are eaten by the 
natives. 
Ear-Shells (aotidz).—The Haliotis is a pearly ear- 
shaped shell, with the outer lip perforated with holes; 
