Key to Subclass Opisthobranchia

Phylum Mollusca
 Class Gastropoda

Taken primarily from  Kozloff, 1987, 1996  (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)
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1a With an external shell large enough to accommodate all ( or nearly all) of the animal when it withdraws 2
1b Either without an external shell, or with a shell that is only partly visible externally and that cannot accommodate more than a small part of the animal 4
2a Shell coiled sinestrally, but otherwise shaped much like that of Tegula, Margarites and other trochid prosobranchs (pelagic; swimming with oarlike flaps of the much modified foot) Order Thecosomata:
Limacina helicina
2b Shell either not coiled or coiled dextrally, and not shaped like that of a trochidprosobranch 3
3a Shell either slender and with a whorl, spire taller than the body or more or less ovoid, or (in Cyclostremella) planispiral and with the diameter greater than the height; aperture not more than half the height of the shell (except in Cyclostremella) Order Pyramidellacea
3b Shell either nearly cylindrical and with a short spire or no visible spire, or narrowly ovoid; aperture at least slightly more than half the height of the shell, and sometimes equal to the height  Order Cephalaspidea
(in part)
4a Pelagic; part of the mantle, just behind the head, elaborated into flipperlike structures used for swimming Order Gymnosomata
4b If pelagic, only temporarily so; lateral extensions of the body, if present, forming sheetlike flaps along much of the length of the foot, and not primarily for swimming 5
5a With a single gill, this located on the right side of the body, between the overlapping mantle margin and the foot [Notaspidean, also called sidegill slugs, have a series of feather-like gills on the right side below the mantle.  They have rolled rhinophores.  Some such as Tylodina have an external shell, while in others such as Berthella the shell forms a thin, inconspicuous plate.  Most species are nocturnal.] [Was identified as Berthella californica in original key] Order Notaspidea: 
Berthella chacei
5b With or without gills, but not with a single gill located on the right side of the body between the overlapping mantle margin and the foot 6
6a With a gill chamber (its length about one-fifth the length of the body) located about halfway back on the dorsum, just to the right of the midline ( with black longitudinal stripes on a green or yellow-green background) Order  Anaspidea:
Phyllaplysia taylori
6b Without a gill chamber on the dorsum (none of the species in our region has longitudinal black stirpes on a green or yellow-green background) 7
7a Shell present, although it may be small and delicate and partly or completely hidden by the mantle; cephalic shield present; fleshy lateral outgrowths of the mantle turned up dorsally but overlapping one another only slightly, if at all.  Order Cephalaspidea 
(in part)
7b Shell absent; cephalic shield absent; lateral outgrowths of the body, if present, forming sheetlike flaps along much of the length of the foot, and if turned up dorsally, overlapping one another to a considerable extent 8
8a Small (maximum length 3 or 4 mm); living in sand, visceral mass distinct from head and foot, and extending posteriorly beyond the foot Order Acochlidiacea
8b Not especially small (commonly more than 3 or 4 mm long, except when young); not living in sand; visceral mass not distinct from the head and foot, and not extending posteriorly beyong the foot 9
9a With a lung, whose opening is on the posteroventral side of the mantle immediately behind the anus (photo); eyes at the tips of distinct stalks (photo) Subclass Gymnomorpha, 
Order Onchidiacea:
Onchidella borealis
9b Without a lung; eyes not at the tips of stalks 10
10a Rhinophores absent [Sacoglossans are mostly small, greenish herbivores, though some species feed on opisthobranch eggs.  Some have shells, while others have no shells but have ridges, folds, or papilli on the back.  Most have rolled, tube-like rhinophores] Order Sacoglossa
(in part)
10b Rhinophores present 11
11a Clavus of rhinophores perfoliate, or with longitudinal ridges, or with vertical pinnate plumes beside the clavus Order Nudibranchia
(in part)
11b Clavus of rhinophores smooth and not distinct form the stalk (in cross-section, the rhinophores may be solid or rolled into cylinders) 12
12a Rhinophores retractile into sheaths Order Nudibranchia
(in part)
12b Rhinophores not retractile into sheaths 13
13a Dorsum with elongate outgrowths, such as cerata 14
13b Dorsum without elongate outgrowths.  Order Sacoglossa
(in part)
14a Rhinophores rolled into cylinders.  Order Sacoglossa 
(in part)
14b Rhinophores solid in cross-section 15
15a Anus on the midline, just posterior to the rhinophores Order Sacoglossa
(in part)
15b Anus on the right side of the body Order Nudibranchia
(in part)

Taxonomic Levels Represented in This Key:
  Order Cephalaspidea
  Subclass Gymnomorpha, Order Onchidiacea, Onchidella borealis
  Order Notaspidea: Berthella chacei
  Order Nudibranchia
  Order Sacoglossa
  Order Anaspidea: Phyllaplysia taylori

Page created by Robbie Wheeling, 7-2002
Edited by:  Dave Cowles 2002-2016
Edited by Hans Helmstetler 1-2003