Phylum Classification
Organisms belonging to the Phylum Mollusca are unsegmented bilaterally symmetrical coelomates which can range in size from microscopic snails to squids which reach 20 m in length. The coelom is limited to a small space around the heart, nephridia, and part of the intestine. Molluscan species possess an open circulatory systems that makes up the principal body cavity as hemocoel. The visceral mass or the organism consists of dorsal epithelium body covered by thick epidermal sheets of skin similar to cuticles. The dorsal mass, or mantle, contains glands which secrete calcareous epidermal spicules, shell plates, or shells. The mantle also forms a cavity which contains stenidia, osphradia, nephridiopores, gonopores and the anus of its complete digestive tract.
The mouth of the complete digestive tract lead to a buccal cavity containing radula to macerate the food to travel from the esophagus to the stomach. The stomach contains one or more ducts which lead to large glandular digestive ceca where extracellular digestion occurs. The intestine continues the intracellular digestion which then leaves the stomach and is expelled through the anus. Products of digestion are absorbed and delivered through the body by the hemolymph. Ctenidia, or gills, and primative ctenidia are used for gas exchange. Cilia on the ctenidia carry water over the gills in the opposite direction of the hemolymph. This creates a countercurrent to enhance gas exchange.
Class Polyplacophora
The Polyplacophora are commonly known as chitons, and are Molluscan species which are flattened elongated. The shell of chitons has unique articulamentum layering of 8 dorsal plates which molluscs with broad ventral foot and 8 dorsal shell plates. The mantle forms a thick girdle which borders the dorsal plates. It epidermis contains calcareous or chitinous spines, scale or bristles for rigidity. The girdle may partially or entirely cover these shell plates. The mantle forms a cavity which encircles the foot and bears 6 to more than 80 ctenidia, or gills, which vary based on species. These organisms lack eyes, tentacle, and crystalline style.
Species Identification
Chaetopleura apicolata
Class Gastropoda
Gastropoda species, commonly known as snails and slugs, consist of approximately 70,000 living species that inhabit marine, terrestrial and freshwater ecosystems. The asymmetrical form of these molluscs contain a single, usually spirally coiled, shell which the body can be withdrawn. In some species of Gastropoda, this shell has been reduced or lost resulting in slug species. The visceral mass and mantle rotates 90-180 degrees on the foot during development. This results in the mantle cavity converting to the anterior end of the organisms, and the guts and nervous system twisting. The mantle cavity houses osphradia, hypobranchial glands, and ctenidia. In terrestrial species the ctenidia are lost and replaced with gas exchange structures. The head of Gastropoda consist of 1-2 our of tentacle, complex radio located within the mouth, and eyes which in some species are often reduced or lost. Movement of Gastropoda are facilitated by the muscular creeping foot. This foot may be modified for the swimming and burrowing taxa.
Subclass Prosobranchia
Prosibranchia is a subclass if Gastropoda which consists of shelled marine snails. The shells of these organisms are spirally coiled, although some species posses a cap or tubular shaped shell. The anterior mantle cavity contains the osphardia, ctenidia, hypobranchial glands, nephridiopores and anus of the complete digestive tract. The head, which is positioned over the mantle cavity contain basal eyes on the tentacles. The mouth may contain a radula depending on the species. As the organisms pulls its body inside of its shell a corneous or calcareous opersulum retracts to close the shells opening, or aperture.
Species Identification
Melongena corona
Slides: The pictures display a live specimen of a Crown Conch and a shell of a conch. In an additional image, a dissecting microscope was used to identify the operculum of the live specimen using 8x magnification. Additionally, a drawn diagram was constructed to show the various anatomical part of the subclass Prosobranchia.
Oliva sayana
Video: The movement of Oliva Sayana is captured. The long siphon can be observed at the anterior end of the organisms as it moved towards the edge of the glass container. The O. sayana uses its muscular foot to glide up the containers surface until the organisms feels the surface of the water. The organism is able to use its foot to flip over.
Subclass Opisthobranchia
Opisthobranchia are the aquatic slugs of the class Gastropoda. Few exist as freshwater, but the majority of species resides in marine habitats. The shell has been reduced, thinned externally it internally or lost altogether. The ctenidia and mantle cavity are also reduced or lost which parallel with the reduction of the shell. The head may possess 1-2 pairs of rhinopores or tentacles.
Species Identification
Bursatella leachii
Video: The movement of Bursatell leachii, is captured using a dissecting microscope under 8x magnification. The anterior end moves past the screen first where the head, oral tentacles, rhinophores and eyes can be observed. If you look closely you can see the parapodium undulating on the dorsal side of the organisms providing the creeping movement.
Slides: Radula of a snail was observed under 4x, 10x, and 40x magnification using a light microscope and can be observed below. Pictures of the live specimen in the video are also presented to give a still look at the anterior end of the organism.
Subclass Pulmonata
Pulmonata are the air breathing terrestrial snails and slugs. The mantle cavity consists of a contractile opening and lung. The ctenidia are lost as they are utilized in the exchange of gas in aquatic environments. As with the other subclasses the body is detorted to various degrees depending on the species.
Diagram
Class Bivalvia
Organisms classified as Bivalves are marine or freshwater species which typically have a shell containing two valves hinges together. There are approximately 20,000 living species of Bivalves. Some common species include clams, oysters, and mussels. Theses organisms are laterally compressed and are held together dorsally by ligaments. The shell closes tightly by strong adductor muscles allowing for defense against predators. Unlike other Molluscan species, Bivalves posses a rudimentary head without radula or eyes. However, eyes may appear on other parts of the body. Respiration is accomplished by the use of a pair of large bipectinate ctenidia. Posterior edges of the mantle may fuse to form inhalant and exhalate siphons. The organism feeds by using labial palps allowing ciliary feeding, microphagous, or suspension feeding.
Species Examples
Class Cephalopoda
Approximately 900 living species of Cephalopods exist in marine ecosystems. The quantity of species have decreased significantly through the evolutionary timeline. A widely known, and the only Cephalopod maintaining an exterior shell, is the nautilus. Linerally chambered shells are usually reduced or lost. Other common species include squids, cuttlefish, and octopi. The body cavity is relatively large in Cephalopods as it contains a closed circulatory system, complete digestive tract, and nervous system. The head contains large complex eyes, and will feed using the radula and bead inside of its mouth. Prehensile arms and tentacles surround the mouth to move prey towards the mouth. Each organism maintains 1-2 pairs of complex nephridia for excretion, and 1-2 pairs of ctenidia located in the mantle’s central pallial cavity. A muscular funnel of the pallial cavity allows for jet propulsion for movement.
Diagram
Glossary of Terms
[1] Brusca, R.C., & Brusca, G.J.(2003). Invertebrates. Sunderland, MA: Sinauer Associates, Inc.
[2] Ruppert, E., Fox, R., & Barnes, R. (2004). Invertebrate zoology : A functional evolutionary approach (7th ed. ed.). Belmont, CA: Thomson-Brooks/Cole.
[3] Anderson, D. (2001). Invertebrate zoology. Melbourne: Oxford University Press.
[4] Meglitsch, P., & Schram, F. (1991). Invertebrate zoology (3rd ed. ed.). New York: Oxford University Press.
Academic Coursework 