Mollusks and Marine Platyhelminthes

Phylum Platyhelminthes

This phylum consists of flat worms. Flat worms can be both free-living and parasitic. Most are marine, while some are freshwater and terrestrial. It is biologically important phyla as it is sees the first appearance of bilateral symmetry and the evolution of cephalization. We will focus on four main classes of Platyhelminthes which are Turbellaria, Monogenea, Cestoda, and Trematoda.

Form and Function

- These animals have a true coelom (acoleomate) and this is what causes their bodies to be dorsoventrally flattened.

- Mouth, Pharynx, and Intestine

- Incomplete digestive tract

- Nervous System composed of two ventral cords

- Excretory and Reproduction systems

- Muscle fibers for locomotion

- Oceli

- Chemical and Mechanical receptors

- Ear-like organs

- Flame Cells

- Cerebral Ganglia

Platyhelminthes Reproduction

Flat worms reproduced both sexually and asexually. Flat worms are nearly all monoecious, and contain complex reproductive organs using penis fencing as means of sexual reproduction. Asexually reproduction is done in the form of fragmentation, fusion, or splitting. Parasitic forms require a host for egg hatching.

Class Turbellaria

The Turbellaria class is the only group of marine free-living flatworms. They are known to live under surfaces (e.g. rocks). They use muscles and a ciliated epidermis to glide or swim. A common species of this phyla is Planarian.

Class Trematoda

This class consists of all parasitic flukes, endoparasitic in vertebrates. They posses sucker and hooks for adhesion, lack/have minimal sense organs. These are important parasites of humans and domestic animals. Subclass Dignea has one intermediate host (mollusk) and definitive host (vertebrate).

Class Monogenea

The monogenean class consists of ectoparasites, mainly of fish. They require a single host and have a direct life cycle. They attach to gills of different fish species and feed on their body fluids.

Class Cestoda

Cestoda are tape worms which are parasites of digestive tracts. They posses no digestive organs, have long-flattened bodies and have a scolex.

Phylum Mollusca

The large and unique Mollusca phylum consists of over 100,000 free-living species in 8 different classes. These organisms posses a true coelom and several advanced systems. These phyla can be marine, freshwater or terrestrial. Most are herbivores, but many are carnivorous. Consists of Gastropoda, Polypacophora, Bivalva and Cephalopoda.

Defining Traits:

-Head-foot: Contains feeding, locomotory and cephalic organs.

-Mantle: Protective structure form by two folds of skin that encloses the mantle cavity and secretes the shell. Secretes the shell and body form.

-Visceral Mass: Contains digestive, circulatory, respiratory and reproductive organs.

-Nervous System containing several pairs of ganglia with connecting nerve cords

-Radula

-Muscular foot (locomotion purposes)

-Complete respiratory system with gills or lungs

-Circulatory system with heart

-Gills

Mollusca Reproduction

An important biological innovation displayed by the Mollusca phylum is that it is the first group to only reproduce sexually, and not asexually. They can be monoecious or dioecious and undergo direct development or metamorphosis.

Class Gastropoda

Gastropods are the most diverse out of the Mollusca phylum. It consists of snails, limpets, sea hares, nudibranchs, slugs and whelks. They are secondarily asymmetrical as a result of torsion. Gastropods undergo torsion in the larval stage and coiling of their shell and visceral mass. As a result of torsion, the organisms left gill, kidney and heart atrium are moved and they are the first organisms able to withdraw head into the mantle cavity. They reproduce by internal fertilization and many produce offspring encapsulated in egg cases/capsules.

Torsion

Class Polyplacophora

Members of this class have a flat, elongated body with a shell composed of 7 or 8 dorsal plates with 60-80 pairs of gills. They have no eyes or tentacles and have a reduced head. Their mantle cavity encircles the muscular foot. Common examples of Polyplacophora are chitons, found mostly in intertidal zones (as seen below).

Class Bivalvia

Bivalves are flat shells composed of two valves. There is no head, a large mantle cavity and very little cephalization in these organisms. Bivalves differ from other classes in the Mollusca phyla as they lack a radula, but have labial palps instead. Common examples include oysters, clams, and mussels.

Class Cephalopoda

The Cephalopoda class consists of marine predators such as- cuttlefish, squid, and octopus. These animals have a closed circulatory system, reduced or no shell and a developed head and eyes. They use jet propulsion through the water as mode of locomotion.

In my Biodiversity Lab, we dissected a squid.