Transcript Mollusks
Mollusks There are over 50,000 known species of Mollusks, which makes them second only to the Arthropods in invertebrate phylum size. Characteristics of Mollusks • Mollusks have three body regions: a head, a visceral mass, and a "foot." • The head contains the sense organs, while the visceral mass contains the internal organs. The "foot" is the muscular lower part of the body. Most mollusks have a shell. • Mollusks also have an extension of the body wall called the mantle. This portion of the animal's anatomy is responsible for secreting the shell. The mantle encloses the mantle cavity which contains the Ctenidia (gills), anus and excretory pores. More characteristics • Many mollusks have a radula, a tongue of sorts, which is rough like sandpaper and is used to rasp away at food. • The radula is made of a hard material called chitin, the same material of which Arthropod exoskeletons are made. • Mollusks have well developed body organs (nervous system, circulatory system, respiratory system, etc.) but lack body segmentation. Different Mollusks • There are seven classes of Mollusks: Monoplacophora, Polyplacophora, Aplacophora, Gastropoda, Scaphapoda, Bivalvia and Cephalopoda. Class Gastropoda • The class Gastropoda (meaning "stomachfoot") contains about 70% of the Molluscan species (around 35,000). These are the familiar snails, limpets, slugs, nudibranchs and abalones. Gastropoda Class Polyplacophora • These creatures have eight plates, made of chitin. • Chitons crawl along rocks and forage for food (mostly algae) using their radulae to scrape the ground foraging for food. • These creatures are extremely slow moving. In a year, a chiton may not move more than ten feet. They can detect the presence of light with primitive eyes embedded in the shell plates. Examples Class Bivalvia • The bivalves (meaning "two-shells") are perhaps the most well known mollusks simply because of their history as a source of food. Clams, mussels, oysters and scallops are all bivalves. There are about 15,000 known species of bivalve, with about 80% of them being marine. Class Bivalvia Class Cephalopoda • This includes squids, and octopuses • They are one of the few animals of this phylum that are excellent swimmers. • The "feet" (usually called arms or mistakenly called tentacles) are attached to the part of the body containing the eyes (the "head") while the rest of the body is out in front of the head. Thus, the body does not connect directly to the arms. More characteristics… • They are similar in internal construction. Perhaps the most obvious difference between most cephalopods and other mollusks is the apparent lack of a shell. • Octopuses have no shell and squids have an internal shell made of chitin. • Cephalopods, therefore, have extremely good eyesight. • This eyesight is well suited for finding prey. Interesting facts • The Blue-Ringed octopus of the South Pacific has a salivary gland which secretes a venom to subdue prey. The venom is so powerful that the bite of this octopus is almost always lethal to a human. Blue Ringed Octopus Class Aplacophora and Scaphapoda • Aplacophora and Scaphapoda are rare and/or extremely deep-water creatures.