TYPE MOLLUSOA. 289 



and situated, as in the Solenogastres, at the bottom of a 

 median ventral furrow, the lips of which correspond to the 

 more dorsally situated mantle-folds of such forms as Chiton, 

 Trachydermon (Fig. 129), etc. In all cases, in the groove be- 

 tween the mantle-folds and the foot a number of gills, pinnate 

 processes of the body-wall, are to be found, in some cases 

 occurring at definite intervals along the entire side of the 

 body, in others {Chitondlus) limited to the posterior part only. 



One of the most characteristic features of the 

 Polyplacophora is, however, the shell, which 

 consists of eight calcareous plates arranged in 

 a longitudinal series along the dorsal surface 

 of the body so that the posterior border of one 

 overlaps the anterior border of the other. 

 The series covers only the median portion of 

 the surface, the more peripheral portions and 

 the outer surface of the mantle-lobes possess- 

 ing a large number of scattered spicules, ^f' '~ "^ 



, . . ^ pleura apiculata. 



plates or granules imbedded m their wall. 



The body-wall has not so definite an arrangement of the 

 muscle-fibres lying below the ectoderm as is the case in the 

 Solenogastres, but, on the other hand, the body-cavity is well 

 developed. Indeed the schizocoelic lacunae play a rather sub- 

 ordinate part in the Chitonidae, as the order is sometimes 

 termed, the enterocoelic cavity (Fig. 128) being very large and 

 divisible into three usually separated parts united by bauds, 

 which indicate the original continuity. One of the parts (c) 

 surrounds the intestine and the digestive gland ; another, \j\ng 

 rather towards the anterior end of the dorsal portion of the 

 body, contains the reproductive cells {ro); while the third part 

 {p), lying dorsally and posteriorly, is the so called pericar- 

 dium. 



The two auricles of the heart are elongated tubes which 

 open about the middle of their length into the single ventricle 

 and also unite together posteriori}', the ventricle, also an 

 elongated tube, again communicating with this united portion. 

 Anteriorly the ventricle is continued into a short aorta from 

 which the blood passes to the lacunar spaces of the schizocoel. 

 Two vessels with distinct walls run longitudinally in the foot. 



