Cuvier, 1797
The largest and in many ways the most successful of the main divisions into which the phylum Mollusca is split. More than 60.000 species in marine, freshwater and in terrestrial habitats. Body asymmetrical. Head well-developed, with eyes and sensory tentacles. Visceral mass rotated through 180° in early embryogeny, a process known as torsion, giving the looped gut and nervous system, and anteriorly situated mantle cavity characteristic of the class. Secondary detorsion occurs in some groups. Shell of one piece, premitively conical, typically spiralled. Ventral foot well-developed, used for crawling, burrowing, and swimming. Feeding modes diverse.
[After Hayward et al., 1990]
Gastropoda that occur in the plankton of the North Sea are the numerous and various small veliger larvae of benthic Prosobranchia and Opisthobranchia (not keyed out at a lower level than subclass), and the adult pelagic snails of the Order Thecosomata, the Order Gymnosomata (both Subclass Opisthobranchia) and Order Mesogastropoda (Subclass Prosobranchia).
The key to pelagic gastropods of the North Sea starts at Page 438: Gastropoda. The following species are included:
Class Gastropoda
Subclass Opisthobranchia
Order Gymnosomata
Family Clionidae
Clione limacina
Clione limacina f. filifera
Clione limacina f. gracilis
Clione limacina f. limacina
Clione limacina f. minuta
Family Pneumodermata
Pneumodermopsis ciliata
Pneumodermopsis paucidens
Order Thecosomata
Family Cavoliniidae
Diacria trispinosa
Clio andreae
Clio cuspidata
Clio pyramidata
Styliola subula
Family Limacinidae
Limacina lesueuri
Limacina retroversa
Limacina retroversa f. balea
Limacina retroversa f. retroversa
Subclass Prosobranchia
Order Mesogastropoda
Family Carinariidae
Carinaria lamarckii
Family Janthinidae
Janthina exigua
Janthina janthina
Order Neotaenioglossa
Family Triphoridae
Cheirodonta pallescens
Family Hydrobiidae
Hydrobia ulvae