The members of this order are defined as follows: "Abdomen well developed, and protected by a shell, into which the whole animal can usually retire. Mantle forming a vaulted chamber over the back of the head, in which are placed the excretory orifices, and in which the branchiae are almost always lodged. Branchiae pectinated or plume-like, situated (proson) in advance of the heart. Sexes distinct" (Milne-Edwards). (See Woodward's 'Manual.')
The order Prosobranchiata includes all the most characteristic members of the Branchiate Gasteropods, and is divisible into two sections, termed respectively Siphonostomata and Holostomata, according as the aperture of the shell is notched or produced into a canal, or is simply rounded and "entire."
The Siphonostomata, of which the common Whelk (Buccinum undatum, fig. 212) may be taken as an example, are all marine, and are mostly carnivorous in their habits. The following families are comprised in this section: Strombidae (Wing-shells), Muricidae, Buccinidae (Whelks), Conidae (Cones), Volu-tidae, and Cypraeidae (Cowries).
The Holostomata, of which the common Periwinkle (Litto-rina littorea) is a good example, are either spiral or limpet-shaped, in some few instances tubular, or multivalve; the aperture of the shell being in most cases entire (fig. 200). They are mostly plant-eaters, and they may be either marine or inhabitants of fresh water. The following families are included in this section: Naticidae, Pyramidellidae, Cerithiadae, Melaniadae, Turritellidae, Littorinidae (Periwinkles), Paludinidae (River-snails), Neritidae, Turbinidae (Top - shells), Haliotidae (Ear-shells), Fissurellidae (Key - hole Limpets), Calyptraeidae (Bonnet Limpets), Patellidae (Limpets), Dentalidae (Tooth-shells), and Chitonidae.
The Dentalidae are often regarded as a separate order of the Gasteropods (viz., Scaphopoda), or, by Huxley, as referable to the Pteropoda. They constitute a lowly-organised group, distinguished by the absence of distinct gills or heart, the imperfect development of the head, and the slender tubular shell, with an aperture at each end.
The Chitonidae and Patellidae are often united into a separate order (Cyclobranchiata), characterised by the generally circular disposition of the branchiae. The former have a multivalve shell, and are stated to have the sexes united.