SINGLE-SHELLED MGLLUSKS. 



6l 



(Fig. 63, /"). 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 (Chitonidce). In these (Fig. 64) the 

 shell is composed of eight transverse plates. The young 



FIG. 64. A gasteropod (Chiton). I. Adult, showing plates. II. Chiton 

 dissected : <?, mouth ; g, nervous ring ; ao, great artery from the heart, 

 aorta ; c, ventricle ; c ', an auricle ; br, left branchiae ; oaf, oviducts. Ill, 

 IV, V. Development of free-swimming young. 



(Fig. 64, III) at first have no shells, swimming about bv 

 '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 {Haliotida). The Haliotis is a pearly ear- 

 shaped shell, with the outer lip perforated with holes: 



