DAVIS : BIONOMICAL CONSIDERATIONS IN GASTROPOD EVOLUTION. 63 



a central pillar, i.e. winds round the centre of gravity, and this effects the 

 greatest economy of the strain necessary for the support of the weight of the 

 shell and the visceral hump. 



We thus at last reach the typical Gastropod with its operculum, conical 

 spiral shell, twisted visceral hump, and antero-dorsal branchial cavity, and it 

 is this type which has spread over the shore and down into the shallow seas. 

 A number of primitive Gastropods, however, have managed to survive amidst 

 less favourable surroundings by adapting themselves to special circumstances. 

 Haliotis, for example, has gone back to the old habitat on the underside of 

 boulders, and this has been associated with a flattening of the shell, and con- 

 sideral)le reduction of its spiral, broadening of the right shell-muscle and the 

 foot, modification of the branchial shell-slit into a series of holes, and so on. 



The Fissiirellid le have also re-established themselves in a similar ha' itat, 

 and in association with the clinging habit have re-acquired bilateral symmetry, 

 especially as regards the shell-muscle and the well-known specializations of 

 the branchial cavity and ctenidia. 



Pleurotomaria has migrated into deep water, where perhaps many of the 

 adaptations found in shoUower water are also of value, though the environ- 

 ment is less favourable except as regards competition of other forms. Retrac- 

 tion also is of less importance. 



In all these somewhat divergent types, and in the less known Scissurelln, 

 both ctenidia have been retained, but in Hiliotis, Scissiirel.'a, and even Pieti- 

 rotomiria, the left gill is larger than the right. Probably the environments 

 of these forms are unfavourable to respiration, necessitating a maximum 

 respiratory surface. 



The Docoglossa have specialized in relation, first to a clinging, and then 

 to an attached habit. In correlation with this, the paired shell-muscle has 

 become a horse-shoe, well suited to pull down the shell so that its entire edge 

 comes into contact with the subjacent rock-surface. At the same time the 

 viscera have been compacted, and the shell has become a conical cap, with 

 at most an apical trace of a median spiral. 



It is possible that the BeUerophofiddae may have been early but twisted 

 Gastropods with a median spiral, or they may have been pre-torsional forms. 

 The great broadening of the shell-mouth in some of them suggests a clinging 

 habit, with its need for a complete cover for the animal. 



Probably before the branchial cavity shifted forwards the efficiency of 

 its flushing arrangements was increased by the development of a sinus in 

 the median posterior part of the shell and mantle, which deepened into 

 a slit corresponding to the opening of the anus. This slit has shifted forwards 

 with the mantle cavity, and undergone well-known modifications in Haiiotis 

 and the Fissurellidae. The slit persits both in Pleurotomaria and Scissureila. 



In forms possessing but one ctenidium, the incurrent stream on the side 

 of the lost ctenidium has of course disappeared, and by movement of the 

 more or less median anus and excretory apertures to the right, they have 



