THE GASTROPODA 141 



organs to the left side, is the result of a torsional movement, which 

 has carried the anus and pallial cavity from an originally posterior 

 to an anterior position and at the same time has twisted the visceral 

 commissure. 



V. BIONOMICS. 



The Gastropoda are essentially aquatic animals, and the more 

 archaic species are marine. Some species are specially adapted to 

 brackish waters. In fresh waters there are found sundry Strepto- 

 neura, viz. certain Neritidae, the Ampullariidae, Paludinidae, Valva- 

 tidae, Bithyniidae, Hydrobiidae, several Cerithiidae, the Melaniidae, 

 Cremnoconchus, and Canidia ; nearly the whole pulmonate group 

 of Basommatophora ; and a single Opisthobranchiate, Ancylodoris. 

 Finally, the stylommatophorous Pulmonates and Halicinidae, Cyclo- 

 phoridae, Cyclostomatidae, and Aciculidae among the Streptoneura 

 are terrestrial. In some forms that live in torrential streams, or are 

 subject to being dried up periodically, the respiration is alternately 

 aquatic and aerial, and the Amphibolidae, Siphonariidae, and Onci- 

 diidae are examples of Pulmonates that have returned to a marine 

 existence. The Gastropoda crawl at the bottom of the water, or 

 on the land, or in a reversed position, on the film of mucus secreted 

 on the surface of the water by the glands of the anterior groove of 

 the foot (Basommatophora, Nudibranchia). The Strombidae are 

 jumpers, and a considerable number of Gastropods are swimmers, 

 e.g. the Heteropoda which swim in a reversed position with the foot 

 upwards Janthina (Fig. 135), the "Pteropoda," Phyllirhoe, Accra 

 (Fig. 147), etc. Some families both of Streptoneura and Opistho- 

 branchia burrow in mud or sand, e.g. the Naticidae, Bullidae, etc. 

 Some genera are more or less sedentary, though able to move from 

 place to place such are Patella and Bathyscicidium but others are 

 completely sedentary when adult, and may be fixed either by the 

 substance of their shells such are Vermetus and Magilus (the latter 

 inhabits corals) or by a calcareous plate secreted by the foot, as is 

 the case in Hipponyx. 



The diet of Gastropoda varies according to the group under 

 consideration. Generally speaking, the carnivorous habit is due to 

 specialisation, often accompanied by the development of a pro- 

 boscis. Various forms of Gastropods live and feed on colonial 

 invertebrates such as Synascidians, Hydrozoa, Anthozoa, and the 

 like, and to a certain degree mimic these forms. Thus Ovula lives 

 on Gorgonia, Pedicularia on Corallium, Lamellaria on Leptodinum, 

 various Nudibranchs on sponges or Hydroids. Some Gastropoda 

 are parasitic, generally in or upon Echinoderms, and belong either 

 to the sub-group Capulidae, in which case they are ectoparasites, 

 and had already acquired this habit in Palaeozoic times (Platyceras\ 

 or to the " Aglossa," that is to say, to the little group formed by 



