430 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The lowest forms of the Odontophora are the Polyplaco- 

 phora., or Chitonidcie, and the Scapliopoda^ or Dentalidce. 

 The bilateral symmetry of the body is completely, or almost 



Fig. 121.— I. Chiton Wossnessenskii. (After Middendorf.) 



II. Chiton dissected to show o, the mouth; g, the nervous rin<:^; ao, the aorta; c, the 

 ventricle ; c', an auricle; br, the left branchiae ; od, the oviducts. (After Cuvier.) 

 IIL, IV., V. Stages of development of Chiton cinereus. (After Loven.) 



completely, undisturbed, while the haemal wall is flat, or near- 

 ly so, and there is no visceral sac. 



The Polyplacophora. — The Chitons (Fig. 121, I.) are 

 elongated, slug-like animals, having the mouth at one end of 

 the body, and the anus at the opposite extremity. A rounded 

 lobe surmounts the mouth, but it bears no eyes nor tenta- 

 cula, and there is no definite head. The edges of the mantle 

 are thickened, but little prominent, so that the pallia! cavity 

 is not much more than an elongated groove, beneath and 

 internal to the thickened edge, which is sometimes beset 

 with setae. In the region in which these setae occur, the surface 

 of the mantle is covered by a thick cuticula. The setae, which 



