.124 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery which may be found creeping on the rocks at low tide, has a 

 jointed shell, and looks like a wood-louse. Examination of 

 the animals that live in these five types of shell shows that 

 they are built on as many plans of structure, and to one or 

 other of these plans all molluscs, except the Palaeozoic 

 Conularida (p. 141), can easily be referred. Therefore the 

 Mollusca are divided into the following five Classes : — 



I. AMPHINEURA, of which Chiton is an example, owe 

 their name to the two nerve-cords that run down each side of 

 the body, which is elongated and symmetrical. The mantle 

 always secretes little plates or spicules of shell-substance. 

 They are divided into two Orders : (a) Aplacophora, which 

 have no shell other than the spicules, and therefore are not 



found fossil ; (b) Polypla- 

 cophora, which have a shell 

 of eight larger pieces, sur- 

 rounded by a flexible girdle 

 formed of the mantle-edge, 

 in which are usually smaller 

 plates or spicules (Fig. 66). 

 Appearing first in Ordovi- 

 cian rocks, they have per- 

 FiG. 66.-Sheil of the liviDg Chiton gig^gj till the present day, 



squamosus ; the iront end is to the . , i • • i i j • 



left. With increasing elaboration 



of the shell, but with no 



changes of sufficient importance to mention here. 



II. GASTROPODA, of which the snail Helix is an 

 example, derive their name, meaning Belly-foot, from the 

 position of the large foot beneath the stomach and viscera, 

 which are contained in a hump on the animal's back ; the 

 surface and folded edges of this hump constitute the mantle 

 that forms the shell. Thus the shell is a cone, sometimes 

 short, as in the limpet {Patella), but generally long, and 

 coiled either in one plane as in the ram's-horn snail 

 (Flanorhis), or spirally as in Helix (Eig. 68, 7). In the 

 common snail it may easily be seen that the edges of the 

 •mantle form a cavity (the pallial chamber) on the right side 

 of the animal ; and into this open the anus and genital duct, 

 which have been brought towards the mouth end of the body 

 by the curving upwards of the viscera into the hump. In 

 many gastropods this twisting of the end of the gut forwards 

 and to the right side has affected other organs and notably 

 the nerve-cords. This affords a basis for dividing the 

 Gastropoda into two Sub-Classes : — 



