2. bilaterally symmetrical or secondarily asymmetrical

5. Mantle with shell glands that secretes calcareous shell

6. Shell creates mantle cavity-houses gonads gills, sensory organs and excretory

structure

7. Heart has atria and ventricle

8. Large muscular foot used for locomotion

9. Buccal region with radula

10. Specialized, regionalized gut

11. Trochophore and veliger larvae

Unfortunately, I could discuss aspects of arthropod biology for at least an entire quarter, but I’ll have to stop somewhere to leave room for some other major groups of invertebrates.

Which brings us to another major phyla-

The phylum mollusca is the second largest group in terms of diversity to the arthropods-constituting at least 50,000 species. In addition, due to their hard parts, there have been an additional 35,000 species of fossil molluscs that have been described as well. Morphologically they are among the most diverse, nonetheless, it is possible to generalize about shared features of these invertebrates.

1) With the appearance of the coelom, the protostomes became more complexly organized, and more abundant. Three main adaptive lines emerged- the annelids, arthropods and molluscs.

Annelids and arthropods are well known to exhibit segmentation and metamerism. Unlike these groups, the molluscs have been regarded as unsegmented. However in 1952 some of the most primitive molluscs have been discovered and they exhibit a metameric design-suggesting that metamerism may also have contributed to the evolutionary diversification of molluscs as well.

Before I go any further with that nice neat evolutionary scenario, it is important to note now that in the last two decades, primary literature has forwarded the notion that molluscs are derived -not from annelids- but from acoelomate flatworms-some taxonomists even suggest that molluscs don’t really even have a coelom at all-We’ll examine this argument in more detail after we survey the various classes of molluscs.

2) Molluscs are bilaterally symmetrical or secondarily asymmetrical. I say asymmetrical because many molluscs undergo two processes during development that render them asymmetrical. One is coiling, whereby the shell coils in various ways, and a portion of the body coils to accomodate within the shell. Often this results in a loss of one or more paired organs such as gills and kidneys. The other process is torsion which involves a rearrangement of the internal organs from their traditional position by rotation of the digestive system and associated organs up to 180 degrees. I’ll address this strange phenomenon later

3) The coelom is reduced and relegated to areas around the gonads, intestines and heart-not too dissimilar to where our own vestige coelom ended up. The fluid-filled space is definitely surrounded by mesoderm-and is therefore a true coelom, but there is considerable debate as to whether this structure evolved independently from the annelid-arthropod line.

4) The organs are concentrated into one area known as the visceral mass or hump. The organ concentration is the result of positioning vulnerable body parts and organs in the safest place relative to the shell. In closed shells, this means the body must be twisted and contorted to fit within it.- So the position of organs is usually intimately related to the locomotion of the mollucs, and the form of the shell.

5) The epidermis is modified to secrete a calcareous shell. This epidermis is called the mantle and the shell may be secreted externally as in most molluscs, or internally such as in cephalopods and terrestrial slugs and sea hares. It serves a protective function and often has retractor muscles associated with it that allow the shell to be pulled down over the body.

6) Often times the mantle secretes a large shell that overhangs the body to some extent. In this cavity that is formed are the gonads, gills (ctenidia), excretory organs, and usually a sense organ called an osphradium-the osphradium monitors the water current and the contents of sediment or volatiles that may be drawn in. This sensory structure is absent in terrestrial pulmonates-the land snails and slugs, and nudibranches. the gills have strong cilia that create the water current into the mantle cavity.

SHOW PAGE 741 BRUSCA-Heart pumping

7) The heart is divided into one pair or more of auricles and a ventricle from which blood is pumped out via the aorta. Thus blood flow through the heart is something of a two-stage process. The circulatory system is an open one-functionally similar to that found in arthropods. Blood flows through arteries to open tissue sinuses that collects and is directed toward the gills by afferent branchial vessels, from the gills, the blood is returned to the heart via efferent branchial vessels.

Cephalopod molluscs, the squid and octopods, have a somewhat different circulatory system-the main difference being that it is closed-not open. This group of molluscs is aberrant in a number of other ways as well and probably represents the most complex and intelligent of the invertebrates.

8) Molluscs have a muscular foot that is used for locomotion. It secretes mucous which reduces friction and creates a surface for gliding. There are cilia that aid in movement. This is one of the reasons why some believe that mollucs evolved from flatworms-since arthropods completely lack external cilia-and so do many annelids. There are many deviations from this general locomotory plan however.

9) The buccal cavity or mouth region has a radula which is a specialized organ for scraping organic material from rocks or other hard surfaces-this may have been one of the reasons for diversification of molluscs since this structure has been modified extensively among molluscs-almost as much as the shell itself.

10) The digestive system is regionalized and specialized.There is a pair of salivary glands, that lubricates the radula, a muscular esophagus. A stomach that has a sorting region with cilia and a chitin lined area. There are digestive glands-similar to a liver in function and a long coiled intestine.

11) Molluscs may be either hermaphroditic or dioecious- primitively, they are diocious, but they produce a trochophore larva similar to annelids (an argument for relatedness to annelids). Later, this trochophore becomes a veliger larva.

The molluscs have evolved different responses to environmental problems than arthropods.

And this has resulted in very different body plans. Although arthropods are extremely diverse, it is fairly easy to identify one as an arthropod. The mollusc body plan is so diverse-it is difficult to talk about a generic mollusc.

SHOW OVERHEAD OF MOLLUSCAN ARCHETYPE

But if you had to....this is what you would come up with....

This represents a generalized molluscan body plan with most of the common features found across the various classes of molluscs. I would like to point out that this is NOT a hypothetical ancestor and shouldn’t be considered as one-only a useful way of conceptualizing mollusc.

Here are some of the major features rolled into a single individual.

Talk about some of the features-orientation of gills etc.

Now lets consider the diversification of this general plan.

SHOW OVERHEAD OF DIFFERENT MOLLUSC TYPES

Molluscs are an extremely diverse group which has caused difficulty in determining their evolutionary relationships with other groups.

Since molluscs are the second most diverse animal phyla, I feel compelled to attempt an explanation for this diversity. Unfortunately, a ready explanation is not available in most textbooks.

I would like to generalize about what contributes to diversification in general.

1)It is a combination of timing-evolving a major evolutionary innovation that allows expansion into a new niche

2)Life history traits that allow for high speciation processes

3)Possessing a characteristic(s) that may have multiple functions.

We’ve talked about the first two previously, but what do I mean by the third one?

Often a single secretory product or specialized tissue layer may substantially contribute to the success of a taxonomic group- by providing functional innovations with a diversity of functions.-Or in other words-are evolutionarily labile.

Such that adaptations are not merely changes in shape but rather different functions belie histological differentiation and physiological distinctiveness. A complex layer of cells responds to environmental pressures and encouraged diversity in choice of habitat and life style.

What do I mean by this? A couple of examples-

The epidermis of arthropods did this through secretion of the cuticle and exoskeleton-with the exoskeleton-a cascade of morphological changes occurred to take advantage of the properties of this outer integument.

Silk glands in spiders were another that drove diversification and functional plasticity. Spider silk is used in a number of ways- egg protection, food collecting, safety lines, dispersal of young by ballooning, a communication medium etc.

For molluscs, the epidermis has been modified to allow for functional plasticity and diversification in a number of ways as well.

The epidermis secretes mucuous which has a variety of functions

-trapping food particles-particularly in bivalve molluscs

-prevents evaporation of moisture in terrestrial and intertidal molluscs

-aids in locomotion by providing a gliding surface

-the slime may cement the animal to the substratum

-attach eggs together

The epidermis also secretes the calcareous shell which has equally contributed to the evolutionary success and diversification of molluscs.

1) Protection from predators

2) Energy conservation in locomotion

3) providing fixed sites for muscle attachment

4) Storage of excess calcium

5) A waste product sink

6) support to raise the body above the sea floor (facilitate food gathering)

7) built-in channeling of water currents

The mollusc epidermis is specialized in a number of other ways as well.

There are patches on the epidermis that are ciliated allowing for locomotion and movement of water currents for feeding, respiration etc.

Although molluscs are often covered with a hard secreted structure, they themselves are soft. The alternation of circular and longitudinal muscles is reduced and other muscles are more prevalent-allowing for more complex movement patterns.

From an evolutionary standpoint, the mollusc body wall is very plastic.

To a far greater extent than in most animals, the body wall is involved in locomotion.

So we know that arthropod and annelid diversity may largely be attributed to metamerism, and the exoskeleton in the case of arthropods,

Mollusc diversity is probably the result of the generality of the body wall and locomotory strucutures, the presence of the radula-a unique feeding structure, mucous production, and probably, above all else, the shell.

The generalized features just mentioned are not always obvious, in that they are modified extensively in response to selective pressures of habitat etc.

SHOW OVERHEAD OF MOLLUSC DIVERSITY.

Since shells are so prevalent among molluscs, I’d like to start with it.

There are some molluscs that are known to secrete spiculose "pseudoshells"

These are radially arranged calcareous rods that are formed within the epidermis. These molluscs have respiration occurring through the epidermis and gills are absent.

They are very small -maximum size is about 2 mm.

It has been suggested that the earliest molluscs must have resembled this type of mollusc-Hedylopsis riseiri.

It is believed that from this early type of mollusc evolved ones with complete shells-like a roof over a house- this would provide obvious protection for a soft-bodied animal.

This causes problems.

The surface area for gas exchange becomes limited.

Consequently, a need arises for respiratory structures.

SHOW OVERHEAD-Chiton

The chiton, believed to be a rather primitive mollusc, shows some of the generalized structural modifications that occur with a shell.

Chitons (Polyplacophorans) have eight plates running dorsally, for some of them, the plates are internal, for others, they are external. These calcareous plates necessitate specialized structures for gas exchange- and you see these along the margins of the body in a furrow that is formed.

These molluscs suction themselves to rocks and feed on algae with their hard radulas. One of the possible adaptive values of the shell is to weather a beating from a pounding surf. This is certainly the case in chitons. If you consider many of the places where there are large amounts of algae on rocks-involves a strong current, or rough water.

The shell must be strong.

SHOW OVERHEAD OF SHELL CONSTRUCTION

The shell is secreted by a specialized area of the epidermis-the mantle.

The mantle is usually on the dorsal body wall and have specialized epidermal glands called-not surprisingly-shell glands.

The shell is usually composed of calcium carbonate. This is a common material-particularly in marine environments because it is so abundant.

The shell is secreted in layers. There is often an organic outside layer-the periostracum. This has a protein called conchin that is also found in the epidermis itself.

There are two layers of calcium underneath-the outer layer is chalky. This is called the prismatic layer.

The underlying layer is the nacreous layer (or lamellar) and is the stuff that pearls are made of. Many molluscs lack this layer in their shells.

Conchin may be used here to bind calcium carbonate crystals together.

The shells are highly variable in color and patterns that one finds.

The functional significance is not always clear-camoflage colors are common-but so are bright ones. It has been found that there are often metabolic wastes embedded within the shell-so to some extent-shell colors often reflect what the mollusc has been eating.

There is an incredible diversity of shell forms. Many of them are of functional significance.

General bivalve shapes serve to cut through sediment.

Small openings such as those in bivalves are for predator avoidance.

Strongly ribbed bivalve shells resist cracking-also to avoid predators.

Long spines along the lateral margins help maintain the mollusc’s position over soft sediment.

Low, cap-shaped shells dispell strong wave forces (like in limpets)

Knobs and hooks on the surface encourage algae growth and invert. colonizing for

In other cases, it appears the intricate sculptured shells are just a by-product of the distribution of shell glands along the mantle.

No matter what the form of the shell-it offers some degree of protection-and an attachment point for different muscles.

The body walls of molluscs tend to be heavily muscularized-circular, diagonal, and longitudinal muscles abound. The outermost layer has a thin cuticle with some sclerotized proteins.

This cuticle does not resemble the cuticle of arthropods in any way-

First, it doesn’t have any chitin in it (with very few exceptions)-and it is ciliated. Arthropods have no external cilia.

There are various gland cells-perhaps the most abundant being mucous glands.

Mucous glands are most abundant on the ventral surface where they lubricate the cilia-much like in flatworms

Various sensory structures also abound on the epidermis.

Below this epidermis are the various muscle layers-sometimes thick.

In most molluscs, there is a space formed between the mantle, and the rest of the body. This space, the mantle cavity, is of great functional significance.

The mantle cavity is used for many things.

The gills are often housed in this cavity-since it offers a space for oxygenated water to flow-and also protection for delicate gills or ctenidia. The gills of molluscs have cilia associated with them and actively create a current.

Bivalves have taken advantage of the mantle cavity to create a water current for filtering food particles through the gills-using them for feeding structures.

The Chiton mantle cavity resides along the lateral margins of the animal-and serves to direct the current backwards through the various gills.

In most molluscs a water current is created and directed through the mantle cavity-to sweep away ammonia and fecal material. The eggs and sperm from the gonopores are also released into this water stream.

In terrestrial molluscs, the mantle cavity is heavily vascularized and serves as a primitive lung.

In cephalopods, the octopus, and its relatives, the shell is secreted internally and the mantle cavity is a large open area that they use for jet propulsion-sucking water in and squirting it out again with strong muscular contractions.

In gastropods, the mantle cavity is situated anteriorly in adult individuals to receive the water as the mollusc moves forward.

Gastropods are the most diverse and successful of the molluscs and undergo several rather confusing developmental processes that need to be explained in more detail.