228 HISTORY OF THE ORMER. 



pods. The commonest of these are the limpets (Patella, &c.), 

 the ormer, and the slit and the keyhole limpets (Fissurella, &c), 

 while rarer forms include Plenrotomaria, whose shells are sold 

 at fancy prices, and a small form named Scissurella. In the 

 common types we find various adaptations to a creeping life on 

 the intertidal shore, or in very shallow water ; and we may 

 suppose that their common ancestor was settling in this domain, 

 and so possessed that flat creeping foot-sole under the body 

 which gives the name Gastrojioda, to the group. 



This ancestor had a cavity above and behind its head, in 

 which were sheltered a pair of gills. This cavity corresponds 

 to the one found behind and below the body of a cuttlefish or 

 Octopus, for these forms, called Cephalopods, are related to the 

 Gastropods. The change of position from the cuttlefish one 

 to that above and behind the head, is one of the most important 

 events in the early history of the animals from which the first 

 Gastropods sprung, but its discussion is beyond the scope of 

 this paper. On either side of the cavity containing the gills, 

 the ancestral animal we are discussing must have been fixed to 

 its shell by a shell-muscle, in a manner best understood by 

 thinking of the front end of the well-known horse-shoe shaped 

 muscle seen in the limpet after removal of its shell. 



The fibres forming the muscle went down into and formed 

 the main mass of the creeping foot-sole and, when they con- 

 tracted, they pulled the shell down over the animal. It is 

 probable that the shell of the ancestral Gastropod curled away 

 from the head, though not in quite the same way as in a 

 modern snail. 



The shells of some of the earliest Gastropods, the long- 

 since extinct Bellerophons, and of some embryos at an early 

 stage, curl away from the head, keeping always in or near the 

 middle line, so that any such shell can be cut into right and 

 left halves ; and it is probable that our imaginary type had a 

 shell of this kind. 



The pair of gills, in the gill-cavity above the head were 

 bathed by streams of water which came in on either side, and 

 went out again along the middle line, clearing waste products 

 on their way. This outgoing current, however, interfered with 

 the incoming ones, and its exit from the cavity was hastened 

 by the presence of a slit in the shell rim, such as is found in a 

 Bellerophon, and in the slit limpets, and also, as we shall see 

 later on, in a modified form in the ormer. 



From these few details of the external features of the 

 supposed common ancestor of the Gastropods we must proceed 

 to find out along what lines evolution has taken place. 



