Chapter 30 : Invertebrates

Phylum Mollusca (Mollusks).

100,000 species. Coelomate, usually marine.

They have a soft body usually with a hard calcium carbonate shell. The body consists of : foot, visceral mass, mantle. The foot is muscular, for locomotion eg snails. Visceral mass contains the internal organs : digestive, reproductive, respiratory etc. The mantle secretes the shell.

Many mollusks have a radula to scrape away at food. Most have an open circulatory system : blood flows into sinuses ( open spaces in the body ) instead of always being in blood vessels. Gills are in the mantle cavity; oxygen is carried by hemocyanin (blue color).

Metanephridia excrete urine through the excurrent siphon. Mainly dioecious, but snails are usually hermaphrodite. Many have a trochophore larva (have a circle of cilia), the same as marine annelids.

Classes of mollusca

 Class Polyplacophora ( chitons )
Marine. Shell is 8 overlapping plates. Flattened body with a broad foot to attach to rocks. Feed on algae using a radula. Dioecious. eg : Katharina

Class Gastropoda ( snails and slugs )
Aquatic or terrestrial. Largest class in the phylum, roughly 50,000 species. The body is twisted during development : torsion. So the anus ends up towards the front above the head. Snail anatomy Gas exchange is through the mantle cavity. Gastropods are usually hermaphrodite. eg : Conus (cone snail) kills prey with neurotoxin in darts.
Helix uses darts in reproduction to hold the snails together. Helix

Crepidula fornicata ( slipper limpet ) forms colonies, and changes gender from male to female as it gets older.

Class Bivalvia ( bivalves eg oyster, clam )
Aquatic, no cephalization (eyes etc all around the body). Bivalves have two shells held together by adductor muscles. They are filter feeders, and have no radula. Gills for gas exchange. Usually dioecious.
May attach to rocks using byssal threads which are very strong, yet flexible. Traditionally these were used as gold thread in tapestry (cloth of gold). Surgeons are now using byssal threads to replace tendons, as they are stronger than the original tendon.

Mussels and clams are used in biomonitoring (checking for water pollution). eg : Venus ( clam )

Class Cephalopoda
Marine predators, with tentacles to catch prey. The head is well developed, with a large brain. They are the most intelligent of all invertebrates. Closed circulatory system. Dioecious. eg : Nautilus : over 90 tentacles. Nautilus anatomy
The shell has separate chambers connected by a siphuncle which can fill the empty chambers with either water or air to adjust the bouyancy.

Loligo ( squid ) has 10 tentacles. Use camouflage ( cells called chromatophores can change color quickly to match the background) and ink glands to escape predators. Squid have a small internal shell, and live deep in the ocean.

Octopus has 8 tentacles and no shell. Octopus live in coral reefs or ocean bottom, usually in shallow water. They use toxins to kill their prey. Blue ringed octopus' have tetrodotoxin which can kill humans. This toxin is actually produced by mutualistic bacteria in the octopus.

Octopus have complex behavior. They can open jars with their tentacles, have a "mental map" of the area around them and learn from observing other octopi.

Phylum Annelida (Segmented worms)

20,000 species. Coelomate, bilateral symmetry. Aquatic or live in soil. The body is divided into repeating segments ( metamerism ) Tubular gut with specialized areas : pharynx, esophagus, crop, gizzard, intestine.

These worms move using circular and longitudinal muscles pushing against a hydrostatic skeleton.

Circulatory system has hemoglobin to carry oxygen. Gas exchange is through the skin, so the skin has to be kept moist.
Excretory system : metanephridia within each segment excrete urine. They are usually hermaphrodite, with no self fertilization.

Classes of annelid

 Class Polychaeta
Mainly marine. They have paddle-like parapodia for locomotion and gas exchange with many setae or bristles on the outside. Tube worms have tentacles lined with cilia to collect food. Polychaetes are often dioecious ( may have part of the body specialized for reproduction : epitoky ) eg Nereis ( clamworm ).

Class Oligochaeta
Live mainly in the soil eg earthworms. They have a smoother body with few setae. Feed on plants, unlike most animals they can digest cellulose. Digestive system often has a fold along the intestine called the typhlosole to increase surface area. Oligochaetes are hermaphrodite : individuals exchange sperm, then produce a slime tube from the clitellum which carries the egg to the sperm. eg Lumbricus ( earthworm ).

Class Hirudinea ( leeches )
Carnivores or parasites. Leeches have no setae ; the body is divided into 34 segments. They have suckers at both ends to attach to the host. Parasitic leeches secrete an anesthetic and hirudin ( an anticoagulant ).

The leech Placobdelloides is an internal parasite of hippos. In other words it lives inside the hippo (it enters the rectum), so that unlike most leeches it cannot be removed by oxpecker birds that rid many African mammals of leeches that attach to their skin.

Leeches cannot digest blood themselves, they have special bacteria ( Pseudomonas ) that digest the blood once it gets into the leeches gut. eg : Hirudo ( medicinal leech ) is now used in microsurgery to reduce swelling. Leeches USA

Last edited October 2009 by David Byres, dbyres@fscj.edu