Making Connections. Investigating Invertebrates. Mollusks and Echinoderms

From 1990-1996 I worked on with the "100 Schools Project." It was California's implementation of the National Science Foundations grant to integrate science content in a new breed of courses. Monte Vista High School was a leader in curriculum development. I was the editor and primary author of a four textbook series. Titled "Making Connections...", it was published from 1996-2001 by the Grossmont Union High School District. The series is now out of print. 

More often than not over the coming months, I'll be reviving this labor of love. I hope you enjoy learning about science in the way I think makes the most sense. Minor editing has been done to the original text. Additions are in highlighted in this color. My titles for the section of the chapter are this color. Most of the diagrams were retrieved in full color from the Internet since the original books were two-colors only. Content in this series is from Volume 4: Making Connections - Integrating the Science of Energy.

Invertebrate organisms have something supporting their bodies besides bones or bony material. They range in complexity from simple multicellular organisms, like sponges, to animals with complex structures and functions, the crustaceans and arthropods.

Many scientists accept a classification system with five kingdoms.

•          Kingdom Monera consists of bacteria and blue-green algae (also known as photosynthetic bacteria).

•          Kingdom Protista includes algae and single-celled organisms (protozoa).

•          Kingdom Fungi includes only the fungi.

•          Kingdom Plantae consists of multicellular land and water plants.

•          Kingdom Animalia is made up of all animals, both with and without vertebrae. Every living thing is classified in one of these kingdoms.

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Week #3

Covered by a Shell

Phylum Mollusca, containing snails, clams, and octopuses, etc., is the next invertebrate phylum. Mollusks are characterized by soft bodies frequently covered by a hard shell. All mollusks have three major body areas:  a head, a "foot," and a visceral hump. The head is where the nervous system is centralized. The foot is a muscular organ associated with locomotion in the animal. The visceral hump contains the intestines and other vital organs.

 Mollusks are divided into three major classes. 

• Class Gastropoda, the stomach-footed mollusks, are named because they appear to crawl around on their stomach. Because these animals have only one shell, they are also known as univalves. Clams, scallops, and oysters belong to 
• Class Pelecypoda, the hatchet-footed mollusks. The muscular foot of Pelecypods can be extended into the sand. When the foot contracts and pulls the animal forward, it looks a little like a hatchet. Pelecypods are also known as bivalves because they have two shells hinged together. 
• Class Cephalopoda, the head-footed mollusks, are the third major class of mollusks. Squid and octopuses are cephalopods. Tentacles seem to split right out of the head of these animals. Cephalopods have no external shells.

In terms of sophistication of design and function, mollusks are the most advanced invertebrates we've discussed to this point. Each class of mollusks has specialized feeding structures. Gastropods have a rasp-like structure called the radula. As Gastropods crawl over surfaces, the radula scrapes away food material which is ingested by the animal. Holes in the center of leaves are evidence of the work of the Gastropods' radula. 

Cephalopods also have a radula. However, the food they eat is captured by the tentacles. The food is brought to a beak where chunks are bitten off. The radula, which lies inside the beak, then grinds the food. 

Pelecypods do not use radulae or tentacles in feeding. The gills of these animals are covered with a sticky mucus. Tiny particles of food are drawn in with the water that brings oxygen to the gills. Food sticks to the mucus. The mucus is pushed into the digestive tract where the food is digested. This method of feeding is called filter-feeding or mucus-feeding.

All Mollusks breathe with gills, even those which are not filter-feeders. Those mollusks which live on land have a moist depression, the mantle cavity, in which the gills are located. Land mollusks cannot live where it is too dry because their gills need moisture in order to function.

Hydraulic-Powered Animals

There is one major invertebrate phylum which is exclusively marine, found only in the ocean. These animals belong to the phylum Echinodermata. Echinoderms have a spiny skin. Small spines, spikes, or nodules stick out of the surface of the animal's body giving them a rough or spiny appearance. Sea stars, brittle stars, sea cucumbers, sea urchins, and sand dollars are the primary examples of members of this phylum.

  

The echinoderms are designed around a central axis. Their body radiates outward, usually in a plan based on the number "5."  It is easy to see the basic echinoderm body plan in the sea star with its central disk and five radiating arms.

Echinoderms move by using a unique body system. While most animals have muscles which move them around, the echinoderms system is hydraulic in nature. The water vascular system is composed of tubes and sacks which are filled with water. Contracting one of the sacks, called ampulla, forces water down into one type of tube, a tube foot. The tube foot extends because of the water being forced into it. A tiny suction cup on the end of the tube foot attaches to whatever it contacts. Contracting of the tube foot forces the water back into the ampulla and pulls the animal in the direction of that tube foot.

  

A sea star or sea urchin can have as many as several hundred tube feet. The direction of movement of an echinoderm is random. The animal goes wherever the majority of its tube feet take it. Of course, sea stars can sense food. When they do, they begin to move, although slowly, in the direction of the food source.

A sea star uses its water vascular system for feeding as well as for moving around. A sea star will grab a bivalve mollusk. Some of the tube feet begin to contract and pull on the shell of the bivalve. While those feet are contracting, other tube feet are relaxing. During this contract/relax rotation, some tube feet pull, while others rest. (Click the link for a quick video of this process.)

The muscle holding the two sides of the bivalve shell together has no chance to rest. The pull against the mollusk's shells is continuous. Eventually, the mollusk muscle gets tired, and the two shells spread open just a small amount.

In a totally gross maneuver, the sea star inverts its stomach—the stomach is pushed outside the sea star body—inside the mollusk shells. Digestive fluid is secreted from the stomach lining. The mollusk is digested inside its own shell. The "mollusk juice" is absorbed by the sea star stomach. This process explains why you sometimes find empty bivalve shell still attached together by the muscle at one end.

Next Making Connections: Investigating Invertebrates. Arthropods –Introduction

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