Artificial Key to Adults of the Free-living Marine Invertebrate Groups Commonly Found in the Pacific Northwest:

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1a Body mass or colony has the appearance of an amorphous mass. May be hard, spongy, or gelatinous. May be a thin or thick crust or may be erect. Often have holes, which may be either single, paired, or scattered and may be on the tips of elevations on the mass. 2
1b Body mass or colony variable, but does not have the appearance of an amorphous mass. 3
2a Mass is soft or hard and may contain calcium or glass spicules. Larger holes, if present, do not occur in pairs. Phylum Porifera: Sponges
2b Mass is gelatinous or leathery and does not contain spicules. Holes are in pairs. A basketlike network may or may not be visible inside. Phylum Chordata, Subphylum Urochordata, Class Ascidiacea: Tunicates
3a Body is gelatinous and the species is pelagic 4
3b The body is not gelatinous or the species is not pelagic 11
4a Radially symmetrical. Have one or more swimming bells that contract for propulsion, or swim by beating rows of comblike cilia. 5
4b Not radially symmetrical 8
5a Swim by beating rows of comblike cilia Phylum Ctenophora
5b Swim by contracting one or more gelatinous bells, but not by beating rows of comblike cilia. Contain stinging cnidocytes. 6
6a Solitary medusae which are often large, often colored (red, yellow, orange, and pink are common colors). The margin of the bell is often scalloped into lappets. No velum is present. Manubrium is drawn out into 4 or 8 frilly oral arms. The gut may be in the form of four pouches lined with colorful gonads, or may have extensive gastrovascular canals. Note: If attached by the exumbrella, or if a stalk protrudes from the exumbrella, see choice 11a, Order Stauromedusae) Class Scyphozoa, Order Semaestomae
6b Solitary medusae or a colony of swimming bells. Not often more than 6 cm in diameter. Usually not colored (may have white). The margin of the bell is not scalloped into lappets but often has a velum. Mouth is in the form of a tubelike manubrium but is not drawn out into frilly oral arms, though its margins may be lobed or frilly. Gut is a small central cavity with (usually four) radiating canals and a ring canal near the margin of the umbrella. A swollen tentacular bulb occurs at the junction of each radial canal with the ring canal. Colonial forms consist of a string of swimming bells, tentacles, and perhaps a gas-filled float 7 [Class Hydrozoa]
7a Solitary medusae Hydrozoan medusae
7b Colonial Class Hydrozoa, Order Siphonophora
8a Swim by beating comblike rows of cilia Phylum Ctenophora
8b Do not swim by beating comblike rows of cilia 9
9a Body barrel-like. Take water in through an anterior buccal or incurrent siphon, filter it through an internal pharyngeal basket, and pump it out a posterior atrial or excurrent siphon Salp or Doliolid
9b Do not filter water by pumping it through a buccal siphon, pharyngeal basket, and atrial siphon 10
10a Laterally compressed, the foot is a ventral fin. Swim upside-down. Have a shell which may be large or small Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Order Mesogastropoda, Superfamily Heteropoda: Heteropods
10b Foot has two fins projecting laterally and perhaps anteriorly or ventrally, used either for rowing or for sculling in a figure eight. May or may not have a shell Phylum Mollusca, Class Gastropoda, Subclass Opisthobranchia: Shelled or naked pteropods
11a Radially symmetrical, a jellyfish which is attached to the substrate by a stalk from its exumbrella Phylum Cnidaria, Class Scyphozoa, Order Stauromedusae
11b Not a jellyfish 12
12a A modified polyp which floats on the ocean surface. Has a chitinous sail. Normally oceanic but may be blown ashore during storms Phylum Cnidaria, Class Hydrozoa, Velella velella
12b Not a modified, floating polyp with a sail 13
13a Growth form is a cuplike polyp, either solitary or as a colony 14 [Phylum Cnidaria]
13b Growth form is not as a polyp 20
14a Small polyps (usually less than 1 mm), a few solitary but mostly colonial due to budding. Colonies may be arborescent (bushlike) or pinnate (featherlike). Not usually brightly colored. Different polyps in the colony are often polymorphic (specialized for different functions). Polyps are usually at least partially surrounded by a proteinaceous coat called a perisarc. Class Hydrozoa, Polyps of Order Hydroida
14b Polyps large and solitary, or if small, not in an arborescent or pinnate colony surrounded by a protein perisarc 15
15a Polyps have eight pinnate tentacles (with featherlike side branches) Class Anthozoa, Subclass Alcyonaria (Octocorallia)
15b Tentacles not eight in number, and not pinnate 16
16a Polyps larger than 1 mm in diameter, with no calcified skeleton. 17
16b Polyps with a calcified skeleton, and may be 1 mm or less in diameter 19
17a Solitary polyps‚€“not connected by a stolon 18
17b Colonial polyps, connected by a stolon Order Zoanthidea:


18a Tentacles taper to a point, no knoblike tips Order Actiniaria: Sea Anemones
18b Tentacles end with knoblike tips Order Corallimorpharia: Corynactis californica
19a Solitary polyps, diameter greater than 2 mm, with mesenteries and tentacles in multiples of 6.  Order Scleractinia: True corals
19b Polyps colonial, polyp diameter may be less than 2 mm. Mesoglea is acellular. Usually purple, calcified colonies of many polyps Class Hydrozoa, Order Stylasterina:
20a Shaped like a round or flattened worm or slug; with a soft exterior (includes animals that may be living in a cemented, calcified, or chitinous tube and soft pelagic worms) 21
20b Does not have a wormlike or sluglike shape and a soft exterior 40
21a Permanently attached to a substrate; exterior is a leathery tunic with two openings, the incurrent (buccal) and excurrent (atrial) siphon Phylum Chordata, Subphylum Urochordata, Class Ascidiacea: Solitary Ascidians
21b Not permanently attached to a substrate nor with a buccal and atrial siphon 22
22a Small, flattened, predatory pelagic worms with lateral fins and long setae for jaws. Phylum Chaetognatha
22b Not small, flattened predatory pelagic worms with lateral fins and long setae for jaws. 23
23a Bodies are clearly segmented‚€“with repeating units 24 Phylum Annelida
23b Bodies are not segmented with repeating units 25
24a Have parapodia on at least some body segments; parapodia usually have many bristly setae. May be freeliving, burrowing, or live in a tube Class Polychaeta
24b Have no parapodia. Rounded worms with multiple segments and a few setae Class Oligochaeta
25a Flat worms with a blind gut; the only opening to which is a pharynx which is generally midventral. The gut has many side pockets, often visible from the outside Phylum Platyhelminthes, Class Turbellaria, Order Polycladida:
Polyclad Flatworms
25b Not flat worms with a blind gut 26
26a Feed by means of a specialized circle of tentacles or ciliated ridges (lophophore) near the mouth 27
26b Do not have a specialized circle of feeding tentacles or lophophore near the mouth 28
27a Deposit or suspension feeders. Feed with a specialized circle of feeding tentacles which are held out in the water or scraped along the sediment, then stuffed into the mouth. Usually have tube feet, which may be in five longitudinal rows along the body. Phylum Echinodermata, Class Holothuroidea: Sea Cucumbers
27b Suspension feeders. Worms which live in a chitinous tube and feed by extending a ciliated ridge or lophophore up into the water. Phylum Phoronida
28a Flattened, predatory ribbon worms, often brightly colored, which capture prey using an eversible proboscis which may have fangs Phylum Nemertea: Ribbon worms
28b Not flattened, predatory worms which hunt with an eversible proboscis 29
29a Round worms which burrow in soft sediments, or live in crevices in rock or coral. Have a large, extensible projection on the head which may or may not be completely retractable into the trunk. 30
29b Not round worms living in sediments and with a large, retractable projection on the head 31  [Phylum Mollusca, Class Gastropoda, (in part)]
30a Anterior structure is a flattened proboscis whose edges fold ventrally to form a gutter which leads to the mouth. Proboscis cannot be completely retracted into the trunk. Trunk has two large, hooked setae on the ventral side near the anterior end Phylum Echiura
30b Anterior structure is a rounded introvert, which is greatly extensible and can be completely retracted into the trunk. Mouth is at the anterior end of the introvert. Does not have two hooked setae on the trunk Phylum Sipuncula: Peanut worms
31a Pelagic, with foot extensions into a ventral fin or lateral paddles for swimming. May or may not have a shell 32
31b Most not pelagic. Have a ventral foot but it does not extend into a ventral fin or lateral paddles for swimming. 34
32a Elongated, twisted cylindrical gastropod mollusk, with a ventral extension on the foot which serves as a fin for swimming. Swims upside down and has a reduced shell Subclass Prosobranchia, Order Mesogastropoda, Superfamily Heteropoda: Heteropods
32b Pelagic gastropod with a foot with lateral extensions used for rowing 33 Subclass Opisthobranchia (in part):
33a Have a shell Order Thecosomata: Shelled pteropods: Limacina helicina
33b Do not have a shell. Part of the mantle or foot, just behind the head, is elaborated into flipperlike structures for swimming Order Gymnosomata: Naked pteropods
34a Rhinophores absent Order Sacoglossa (in part)
34b Rhinophores present 35
35a Clavus of rhinophores perfoliate, or with longitudinal ridges, or with vertical pinnate plumes beside the clavus Order Nudibranchia (in part)
35b Clavus of rhinophores smooth and not distinct from the stalk (in cross-section, the rhinophores may be solid or rolled into cylinders) 36
36a Rhinophores retractile into sheaths Order Nudibranchia (in part)
36b Rhinophores not retractile into sheaths 37
37a Dorsum with elongate outgrowths, such as cerata 38
37b S
Dorsum without elongate outgrowths
Order Sacoglossa (in part)
38a Rhinophores rolled into cylinders Order Sacoglossa (in part)
38b Rhinophores solid in cross-section 39
39a Anus on the midline, just posterior to the rhinophores Order Sacoglossa (in part)
39b Anus on the right side of the body Order Nudibranchia (in part)
40a Less than 1 mm long, meiofauna (between sand grains or in mud), with a spiny cuticle divided into plates. With an anterior protrusible head. Phylum Kinorhyncha
40b Not an animal less than 1 mm long, with a spiny cuticle divided into plates and an anterior protrusible head. 41
41a Body largely or entirely enclosed within two external ‚€œvalves‚€ or shells which hinge together on one side. Neither body nor any appendages are segmented. 42
41b Body is not enclosed within two external valves which hinge together on one side. Body or appendages may or may not be segmented 43
42a Body is composed of a large gill used for filter feeding, a foot which can protrude from the shell, a visceral mass, and a mantle which secretes the shell. Foot is used for digging or for secreting proteinaceous byssal threads which are used for attachment to the substrate. Valve opening is limited by muscle action. Do not attach to the substrate by means of a fleshy stalk which protrudes from the shell. Phylum Mollusca, Class Bivalvia
42b Valve opening is limited by muscle action or by interlocking parts of the valves (in which case the valves cannot be opened by more than a few degrees without breaking them). Filter feed using a lophophore. Do not have large, separate gills nor a protrusible foot. Attached to the substrate by a fleshy stalk which protrudes from one of the valves. Phylum Brachiopoda:


43a Pelagic, with a tadpole-like body which secretes a gelatinous "house" used for filter feeding. Currents through the house are generated by beating of the tail Phylum Chordata, Subphylum Urochordata, Class Larvacea:
43b Not pelagic and tadpole-shaped, living in a gelatinous house 44
44a Body segmented. Covered with an exoskeleton, and having jointed legs 45  [Phylum Arthropoda]
44b Body unsegmented 59
45a Spider-like, with long-legs extending from a small cephalothorax and with sucking mouth parts. No antennae but have a proboscis. Either large and found in deep benthic habitats or less than 1 cm and usually found on sea anemones Class Pycnogonida
45b Have jaws instead of sucking mouth parts. Have antennae. Not usually spider-like 46
46a Small (< 1 cm long) as adults, pelagic. No special gills but legs are leaf-like appendages used both for swimming and for respiration Class Branchiopoda
46b Small or large, but do not breathe with leaf-like appendages used both for swimming legs and for respiration 47
47a Small (most < 1 mm) as adults (except for some bizarre parasitic forms). Short, cylindrical body. One median naupliar eye but no compound eyes. Abdomen has no appendages, except that the anal segment is forked into two ‚€œcaudal rami‚€. Long, conspicuous, uniramous first antennae Class Copepoda: Copepods
47b Not with a small, cylindrical body, one median naupliar eye, and caudal rami 48
48a Permanently cemented to substrate as adults. Either surrounded by six calcareous plates which are cemented to the substrate or attached by a fleshy stalk and covered with a set of plates which do not cement to the substrate. Feed by beating filamentous legs or holding them in the current 49  [Class Cirripedia, Order Thoracica: Barnacles]
48b Not permanently cemented to the substrate as adults. 50
49a Surrounded by six calcareous plates which are cemented to the substrate Suborder Balanomorpha: Acorn Barnacles
49b Attached by a fleshy stalk; body is surrounded by plates which are not cemented to the substrate Suborder Lepadomorpha: Goose Barnacles
50a Possess a cephalothorax; the head and several or all the thoracic segments are fused to one another at least dorsally by a carapace. Most or all individual thoracic segments are not visible from the dorsal side 51
50b The head and thoracic segments are not fused together with a carapace. The individual thoracic segments are visible from the dorsal side 57
51a Shrimplike, lobster-like, or crab-like; commonly found. If shrimplike, the dorsal carapace is attached to all thoracic segments. Females carry eggs on the thoracic or abdominal legs, but do not carry them within a special thoracic pouch (marsupium) formed by inner branches from the legs. 52
51b Shrimp-like, usually small (intertidal species are usually < 3 cm long), and not commonly found. Dorsal carapace not attached to all the thoracic segments, though it may overlap them. Females carry eggs within a special thoracic marsupium formed by the inner branches of the legs 56
52a Crablike: The thorax is covered with a wide carapace, the first pereopod is chelate (a pincer), the abdomen is held tucked tightly under the thorax and has no uropods. All ten legs are easily visible and functional for locomotion. The eyes are usually lateral to the large second antennae. Infraorder Brachyura: True crabs
52b Crablike or shrimplike, but if crablike the fifth pair of legs are reduced or turned upward and eyes are medial to the large second antennae. 53
53a Clearly shrimplike or lobster-like: Thorax is laterally compressed and usually longer than wide; abdomen is extended behind the thorax. 54
53b Crablike, with a wide, depressed thorax or somewhat shrimplike, with a globular or slightly elongated thorax. Fifth pereopod (walking leg) reduced or turned upward, not useful for walking. Abdomen may be held under the thorax, but more loosely than in the true crabs; may be extended to the rear somewhat like a shrimp, or may be soft and twisted (in hermit crabs). Abdomen usually has uropods. Infraorder Anomura: Porcelain crabs, Hermit crabs, Lithodid crabs, Squat lobsters
54a Shrimplike. Gills are dendrobranchiate (tree-shaped, with two main branches from the main stalk and many compact side branches). First three pairs of legs are chelate but not with enlarged chelipeds. Pleuron of second abdominal segment does not overlap that of the first and third segment. Females do not brood their eggs. Suborder Dendrobranchiata: Penaeid prawns
54b Shrimplike or lobster-like. Gills are phyllobranchiate (a series of stacked hollow plates) or trichobranchiate (a series of hollow tubes attached to a common stalk). Females usually brood their eggs. 55
55a Shrimplike: Abdomen is as high or higher than it is wide. Pleuron of second abdominal segment overlaps that of the first and third segment. First three pairs of pereopods (walking legs) are either chelate or subchelate and the first or second pair usually has enlarged chela (though often much smaller than those seen on crabs). Phyllobranchiate gills.  Infraorder Caridea: True shrimp
55b Lobster-like: Abdomen is wider than it is high. Trichobranchiate gills. First two pereopods chelate, first usually much enlarged. Usually live in mud burrows or in rubble. Infraorder Thalassinidea: Ghost and mud shrimp
56a Possess a distinctive, bulbous, enlarged head and thorax with two anterior extensions that swing together to form a false rostrum. Abdomen narrow and has slender, elongated uropods at the end (no tail fan). May or may not have eyes. Burrowing filter feeders Order Cumacea
56b Shrimplike, with a laterally compressed thorax, and usually have a true rostrum. Most intertidal species have no gills. Females may have reduced pleopods. Fourth pleopod may be enlarged in the male. Possess stalked compound eyes. Freeswimming or benthic. Order Mysida: Opossum shrimps
57a Small (mostly 1-2 mm long), small carapace covers only first and second thoracic segments. First thoracic appendages are maxillipeds and second pair are large, subchelate gnathopods. No eyes, or if eyes are present they are on the ends of immovable stalks. Order Tanaidacea: Tanaidaceans
57b Large or small. With no carapace, though the head may be fused to one or more thoracic segments. One pair of maxillipeds and seven pairs of uniramous pereopods. The coxae of the pereopods usually expanded into coxal plates which make the thoracic segments appear to be wider or higher than they actually are. Compound eyes sessile, absent, or on the ends of immovable stalks in one group. 58
58a  The terga of thoracic segments usually extend laterally, and the coxal plates of the pereopods often extend even farther laterally so that thorax appears wider than it is high (dorsoventrally flattened). First pereopod may or may not be subchelate. Well-developed pleopods, used for gas exchange. Telson is fused with one or more of the 6 abdominal segments, forming a pleonite.  Order Isopoda: Isopods
58b The first one, two, or three pereopods frequently chelate or subchelate gnathopods. Coxal plates of pereopods usually extend ventrally so that thorax appears higher than it is wide (laterally compressed). Gills are thoracic. First three abdominal segments have pleopods, last three have uropods. Telson is free or fused with the last abdominal segment. Order Amphipoda: Amphipods
59a Highly motile animals, bodies soft and covered with a mantle; have well-developed eyes and 8-10 arms or tentacles Phylum Mollusca Class Cephalopoda: Octopus, squid
59b Do not have a soft body covered with a mantle, with both well-developed eyes and 8-10 arms or tentacles 60
60a Body composed of a central disk with (usually 5 or more) large projecting rays. Body surface may be hard, membranous, leathery, or spiny  61
60b Body not composed of a central disk with large projecting rays.  62
61a Body surface leathery or spiny, rays thickest at base and taper gradually into central disk. Underside of rays has a well-developed ambulacral groove with tube feet. May have up to 25 or more rays. Move by attaching and reattaching tube feet. Phylum Echinodermata, Class Asteroidea: Sea Stars
61b Body surface hard or membranous, rays composed of interlocking ossicles and usually adjoin abruptly to central disk without tapering. Usually no more than 5 rays, though they may branch repeatedly in basket stars. No ambulacral groove on underside of rays. Move by lifting themselves along by the rays. Phylum Echinodermata, Class Ophiuroidea: Brittle Stars
62a Body round or oval, and surrounded by rigid interlocking plates which form a globular or platelike ‚€œtest‚€. Covered with large or small movable spines and tube feet. Phylum Echinodermata, Class Echinoidea: Urchins and Sand Dollars
62b Body not composed of a rigid test of interlocking plates covered with movable spines and tube feet 63
63a Attached, colonial, composed of many tiny individuals (nearly microscopic) each of which is encased in a surrounding calyx, feeds with a lophophore, and has a U-shaped gut. Colony may be erect and bushlike, leaflike, or a flat crust 64
63b Not a colony of nearly microscopic individuals feeding with a lophophore and with a U-shaped gut  65
64a Anus is on an elevated anal cone which is surrounded by the tentacles of the lophophore (only a few species) Phylum Entoprocta
64b Anus open outside the tentacle ring of the lophophore. Colonies may be erect, branching, leaflike, or encrusting, and may be calcified. Individuals may be boxlike, oval, vaselike, or tubular in shape. Very common and abundant fouling organisms Phylum Bryozoa (Ectoprocta): Bryozoans
65a Sediment dwellers. Having an elongated, tapering, tusk-like shell, open at both ends, one end of which protrudes from the sediment. Phylum Mollusca, Class Scaphopoda: Tusk Shells
65b Having a single coiled, cap-shaped, earlike, or irregularly tubelike shell of calcium carbonate Phylum Mollusca, Class Gastropoda: Shelled Snails

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