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. BIroNoMIcs. 
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,” Phyllirhoé, Acéra 
(Fig. 147), etc. Some families both of Streptoneura and Opistho- 
branchia burrow in mud or sand, e.g. the Naticidae, Bullidae, ete. 
Some genera are more or less sedentary, though able to move from 
place to place—such are Patella and Bathysciadiwm—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 certaindegree mimic these forms. Thus Ovula lives 
on Gorgonia, Pedicularia on Corallium, Lamellaria on Leptoclinum, 
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 
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