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The Animal Clade
Chordata
Ancestral Choanoflagellate
Cnidaria
Hemichordata
EchinodermataExtant
deuterostomia
Arthropoda
Annelida
Mollusca
protostomia
coelomates
Nematoda
Rotifera
pseudo-coelomates
Platyhelminthes
acoelomates
radiatabilateria
eumetazoa (true tissues)
Porifera
parazoa
loss of chloroplast, colonial organization
This cladogram omits several smaller animal phyla!
Animals
Domain EukaryaKingdom AnimaliaPhylum Mollusca
35,000 species making this the second-largest phylum of Animalia
Polyplacophora: chitonsThe most-primitive mollusc has 8 valves (plates) protecting its soft tissues beneath. The chiton foot attaches to rocks and the animal uses its radula to scrape organic material from the rock surfaces.
The chiton has multiple eyes. Some are just light-sensitive spots.The primary eyes are of a lens-type. Many chiton species lack eyes.
mouth
radula
valve plates
gonadheart
pericardial cavity(coelom)
mantle
anusfootdigesti
ve gland
nephridiumstomach
ventral nerve cord(not shown)
This cartoon shows a longitudinal slice of a chiton with the three principal parts: foot (locomotion or attachment), visceral mass (internal organs), and mantle (secretes valves).
As for all other molluscs, chitons use a radula to scrape their food from environmental surfaces. Below is a radula removed from a chiton mouth.
http://www.zetnet.co.uk/~pm/photos/snail.jpg
Gastropoda: snails and relatives (slugs)Snails have a single spiral-shaped valve (univalve)Slugs and nudibranchs have lost this feature.
foot
shell
eye
optical tentacle
sensory tentaclesgonopore
http://www.zetnet.co.uk/~pm/photos/snail.jpg
And now for a look inside our gastropod mollusc…
The shell obviously provides a hard covering for the visceral mass.The snail shown here is a pulmonate, with a vascularized mantle cavity serving as a lung. Vascularizing this led to loss of the gills in most gastropods.
The gastropods, are clearly hermaphroditic, and some are self-fertile.
These two slugs are showing mating behavior.The slugs are dangling on a slime thread and grip each other with their feet.The slugs evert their reproductive organs out through the gonopore.The organs unite and spermatophores are exchanged.Sperm are stored in a spermatheca for a week or more. Syngamy and deposition of zygotes occurs later.
http://users.actrix.com/littlejn/bivalve.jpg
http://www-biol.paisley.ac.uk/biomedia/
graphics/jpegs/aopercu.jpg
Bivalva: bivalves
This group includes the clams, oysters, mussels, and scallops.
Their body is typical mollusc too, but with two hinged valves (shells)
Here is a cartoon of a lateral view of the foot, visceral mass and mantle
Adductor muscles to hold the valves together.Bivalves have gills rather than lungs.Their incurrent siphons take in plankton lodging in mucus.
The mucus laden particles gather on the gills (palps) and enter the mouth.
The mouth lacks the radula.
This cartoon is shows a plane of section perpendiular to the previous one.
The foot can push a bivalve through sediments.
The food-trapping gills are used for gas exchange.
The heart pumps the blood into the hemocoel bathing the tissues. It goes through the gills for gas exchange. The blood then returns to the heart.
Nephridia cleanse the blood of nitrogenous waste.
hinge and ligament
nephridium
mantle
shell
gills
foot
gonad
intestine
heart
http://www.photogg.de/frokt02/10-10-scallop.jpg
Here are three different molluscs. Between the valves of the bivales the mantle fringe is quite visible. With the valves ajar, the bivalve can carry out its filter feeding. If you swim nearby, the bivalve adductor muscles snap the valves shut.
Contrary to the filename, this is a Humboldt squid. It is certainly large, but is not the giant squid.Between the tentacles part of the beak is shown.The eyes face the man’s knee and elbow.The mantle is in his lap and the fin is over his shoulder.
Another cephalopod is the octopus.It obviously has eight tentacles surrounding the mouth…no, duh!This one is obviously swimming.
http://www.pithagorio.net/Mat/octopus%202.jpg
Here is another swimming octopus. The idea of cephalopod (head-foot) is shown nicely here. Behind one tentacle the siphon is showing the basis for jet-action locomotion among cephalopods.