2.4 Taxonomy: Mollusca-Echinodermata
PHYLUM MOLLUSCA
Molluscs are triploblastic, coelomate animals with FOUR basic characteristics:
•
a FOOT
•
a SHELL generally secreted by the mantle
•
a HEAD
•
a RADULA = chitinous ribbon with rows of teeth
The first two classes are small, no representatives; their forms are somewhat degenerate, more worm-like. The mollusks
are a large, diverse group with 150,000 living species, an excellent fossil record (presence of a hard shell), a soft,
unsegmented body and a reduced coelom (but a thick, muscular body wall). They are a good example of adaptive
radiation because they have adapted to exist in nearly every habitat.
CLASS POLYPLACOPHORA
= the CHITONS have a shell reduced to 8 dorsal, hinged plates, marine animals found in the intertidal. See preserved
specimens and plates.
CLASS SCAPHOPODA
= the TUSK SHELLS (named because they are shaped like an elephant tusk), burrowing marine mollusks with a one
piece shell open at both ends. See animal shell
CLASS GASTROPODA
= the SNAILS & SLUGS. Animals with a long, flat foot and a distinct head, a dorsal visceral mass housed by a spiral
shell, some with very bright colors (either terrestrial or marine nudibranchs). Most species have a single shell or no shell.
Largest and most successful class of mollusks because they are found in all environments (terrestrial, freshwater and
marine).
CLASS BIVALVA
= the CLAMS, MUSSELS, OYSTERS AND SCALLOPS. Bivalves have 2 hinged shells and are typically filter feeders
(reduced radula). They are generally sessile (though scallops can swim in short bursts to evade predators) and of
economical and environmental (Musselwatch program to test for environmental contamination nationwide) importance
See preserved and dried specimens and slide of the larval stage: glochidia, a parasite on fish gills, feeding on blood.
CLASS CEPHALOPODA
= SQUID, CUTTLEFISH, NAUTILUS and OCTOPUS. Head and foot merged (name means “head foot”), internal shell
and a large head with conspicuous eyes. Most advanced mollusks with large size span (2cm to 20m = Architeuthis the
giant squid) and very active predatory lifestyle.
PHYLUM ANNELIDA
= the SEGMENTED WORMS. These worms display metamerism (body is segmented into sections or metameres), are
advanced over previously discussed worms (i.e. coelom, closed circulation, CNS and well developed muscle system) and
are generally errant or sedentary. Annelid classification is based on characteristics of setae (bristles on parapodia),
presence/absence of the clitellum (thick, glandular segment) and mouth parts (jaws).
CLASS POLYCHAETA
= marine segmented worms with appendages called PARAPODIA, a well developed head but no clitellum. Generally
dioecious (hermaphrodites, adult animals containing both sexes) with a trochophore larval stage.
CLASS OLIGOCHAETA
= BRISTLEWORMS, fresh water, terrestrial, and a few are marine. The EARTHWORM is a representative example. No
head structures or parapodia but a clitellum. Generally monoecious (separate sexes) with direct development and no larval
stage.
CLASS HIRUDINIA
= the LEECHES. Small group of dorsoventrally flattened freshwater animals (CAUTION – Don’t confuse them with the
trematodes, leeches unlike flatworms have true segmentation) with 2 suckers: one anterior around the mouth and one
posterior.
PHYLUM ARTHROPODA
= meaning “joint foot”, a group including the INSECTS, ARACHNIDS AND CRUSTACEANS. Most diverse, abundant
and successful group of animals (estimated species diversity of 10-30 million, with only 1 million described).
Characteristic jointed appendages (see name), either biramous (2 branches, in marine arthropods) or uniramous
(unbranched), and a jointed chitinous exoskeleton divided into functional body regions (tagma). They are classified based
on mouth parts and appendages.
SUBPHYLUM CHELICERATA
= recognized by the first 2 pair of appendages (first pair = CHELICERAE; the second pair = PEDIPALPS– pincer-like)
and their lack of antennae and mandibles.
CLASS MEROSTOMATA
= the HORSESHOE CRABS. All marine (Gulf of Mexico and Atlantic only), used for medical research (chitin is
used in bandanges and increases healing capacity by 35-50%), adults live as monogamous pairs. Contain a pair of
lateral compound eyes and 1 naupliar eye. See dried specimen.
CLASS ARACHNIDA
= the SPIDERS, MITES, TICKS & SCORPIONS. Animals are mainly terrestrial and none are marine.
Characterized by head and thorax fused to form a cephalothorax and 6 paired appendages (4 pairs of walking legs,
1 pair of pedipalps = used for reproduction and 1 pair of feeding appendages = chelicerae). See preserved
specimens and models.
SUBPHYLUM CRUSTACEA
Crustaceans have two pairs of ANTENNAE and MAXILLAE and only one pair of MANDIBLES (crushing or grinding
mouth organs). They are mainly aquatic with biramous appendages We will consider two classes:
CLASS CIRRIPEDIA
= the BARNACLES. All marine and of economic importance (attach to the hulls of ships causing friction and
therefore reducing ship speeds). They are sessile, living in rocky intertidal areas and contain thoracic appendages
(CIRRI) modified for feeding. They are hermaphrodites but cross-fertilize.
CLASS MALACOSTRACA
= the CRABS, LOBSTERS, CRAWFISH, SHRIMP, KRILL, AMPHIPODS & ISOPODS. This class
encompasses more than half of the crustaceans. Most animals are marine, few terrestrial and some are of
considerable commercial value. They have 10 pairs of WALKING LEGS that will regenerate if lost. The first pair
are often modified as claws or pincers used for crushing invertebrate shells and for grabbing & holding.
SUBPHYLUM UNIRAMIA
All terrestrial with unbranched appendages (UNIRAMOUS) and a single pair of antennae. Respiration (oxygen delivery)
through spiracles and tracheal tubes.
CLASS DIPLOPODA
= the MILLIPEDES. Animals have a cylindrical body with a distinct head and 2 pairs of walking legs per
segment. Most are blind and harmless, but some may secrete chemicals that can burn human skin
CLASS CHILOPODA
= the CENTIPEDES. Animals are segmented with 1 pair of legs per segment (50-100 segments) and like dark,
damp environments. Produce venom in a gland near the front legs. Most specimens are not harmful to humans,
with the exception of Scolopendra which contains a venom of acetylcholine, histamine and serotonin and can
cause serious swelling and fevers but is rarely deadly
CLASS INSECTA
= the INSECTS. Animals with 3 body regions (TAGMATA): HEAD, THORAX, & ABDOMEN, 3 PAIRS of
LEGS and often 2 PAIRS of WINGS in adults. Many insects show increased glycerol (ANTIFREEZE)
production during the winter. This class is the contains the largest and most successful group of animals.
PHYLUM ONYCOPHORA
These are intermediates between annelids and arthropods, wormlike animals mainly found in tropical rainforests:
•
elongate, worm-like bodies without segments
•
legs, but not jointed
•
no cuticle/exoskeleton
•
there kidney is more like worms than insects
PHYLUM TARDIGRADA
= the WATER BEARS (name means “slow walker”). These animals are small and aquatic with legs that legs are not
jointed. They live as members of the plant community mainly on the juices of plants and are considered to be POLYEXTREMOPHILES (i.e. survive temperatures of up to 303ºF, the vacuum of space, for a decade without food and with
1000 times more radiation than cockroaches)
PHYLUM BRACHIOPODA
= the LAMP SHELLS. Solitary animals (all marine and most are less than 5mm) that appear similar to bivalves, because
their body is enclosed in 2 shells. To distinguish them from bivalves look at the symmetry between the shells.
PHYLUM ECHINODERMATA
= name means “hedgehog skin”. Echinoderms seem to be taking a step back - they show radial symmetry rather than
bilateral symmetry, have no defined head and their symmetry is based on 5 branches: PENTAMEROUS. They do have an
endoskeleton made of small plates, generally fused into a TEST and a large enterocoelous coelom which forms into the
water vascular system.
CLASS CRINOIDEA
= the SEA LILLIES or FEATHERSTARS. Often sessile with a stalk and tentacle-like arms that capture food. Feather
stars can detach from their stalk and crawl around.
CLASS ECHINOIDEA
= the SEA URCHINS, SAND DOLLARS, etc. Ossicles are bonded tightly to form a rigid TEST. They are opportunistic
feeders that move using TUBE FEET (arranged in 5 paired rows) and movable using spines.
CLASS HOLOTHUROIDEA
= the SEA CUCUMBERS. Elongated organisms living on the sea floor, extracting nutrients from sediments. No spines or
rigid endoskeleton, instead a flexible body wall containing mutable of catch collagenous tissue (regeneration) and
calcareous plates. When disturbed by predators they can discharge pasts of their respiratory and digestive tract.
CLASS ASTEROIDEA
= the SEA STARS or STARFISH. Animals show have 5 or more arms that are not differentiated froma central disk. They
also move via tube feet confined to ambulacral grooves on the oral side (terminal tubefeet contain photoreceptors = eye
spots). Prey on bivalves, and they use their arms, stomach and tube feet to pry open the shell.
CLASS OPHIUROIDEA
= the BRITTLE STARS have 1 distinct central disc of pentagonal shape and arms clearly differentiated from the central
disk, no abmulacral grooves and reduced or absent tubefeet. Dried and preserved specimens.