The anterior and posterior adductor muscles being cut so that the valves can be pulled apart. The mantle is a membrane that surrounds the internal structures and is characteristic of all mollusks. The portion of the mantle from the exposed surface may have remained attached to the valve that was removed or it may be covering the internal structures.
Find the mantle and if necessary, remove it to expose the internal structures of the clam. Identify the foot, visceral mass, gills, and labial palps. Food is trapped by mucus on the gills and moved by cilia to the mouth. Cilia on the labial palps also direct food and mucus to the mouth. The heart can be found in the area dorsal to and slightly posterior to the visceral mass. The coelom of bivalves is reduced and limited to the area surrounding the heart.
Because the coelom is reduced, it is difficult to see the digestive organs. Cut through the visceral mass and identify the intestine. In the photograph below, the foot and visceral mass have been lifted so that the mouth can be seen. Notice the labial palps on either side of the mouth. Cephalopods are predators and live in a marine environment. A closed circulatory system allows them to move rapidly in pursuit of prey.
They move by jet propulsion; water in the mantle cavity is squirted rapidly through a siphon. The foot has evolved into tentacles around head. Cephalopods have a powerful beak-like structure to tear apart prey.
The sense organs of cephalopods are well developed. Mollusks are the simplest animals with eyes. Some mollusks have lenses and therefore are capable of forming clear images. The camera-type eyes of some cephalopods squid, octopus are capable of focusing and forming clear images.
Cephalopods are fast-moving predators and well-developed camera-type eyes help them catch prey. Well-developed brains especially in octopuses give them a high learning capacity. Cephalopods can hide from enemies by releasing a dark colored fluid from ink sacs. The shell of a nautilus encloses the animal.
Octopuses do not have shells. Examine representative cephalopods on display. Skip to main content. Module 8: Invertebrate II. Search for:. Reading: Mollusks This laboratory exercise covers the following animals.
The mantle and foot can be seen in the figure 1. The visceral mass is underneath the gill. Figure 1. Figure 2. Left: chiton, dorsal surface. Right: ventral surface. Figure 3. A slug. Slugs do not have shells. Figure 4. Adductor muscles of a clam. Figure 6. Figure 7. Figure 8. In a few forms, especially those with a large byssus, the median area may be depressed to form a median sulcus. Two important features are found on the dorsal margin. Anterior to the beaks there may be a defined, often heart shaped area, the lunule.
Posterior to the beaks there may be a similar structure which is usually elongate in outline and depressed, the escutcheon. In many bivalves the valves do not meet at all points around the margins.
These gapes vary in position and most commonly occur posteriorly where it reflects the presence of large siphons. An anterior or anteroventral gape is usually for the extension of the foot and is referred to as the pedal gape. Many bivalves use a byssus extruded from the foot to attach themselves to their substrate. If the byssus is large there is often a ventral or byssal gape. Scallops and scallop-like forms such as Lima have shells that possess ear like extensions, auricles , of the dorsal margin.
In Pteria the auricles may be greatly extended into wings. In scallops the valves remain on the right and left sides although the animal lives in the opposite plane lying on one valve. The byssus is accompanied by a notch in the right valve, and often with a series of small teeth, the ctenolium , along the ventral margin of the notch. In strongly inequilateral shells the posterior dorsal edge can be further divided. In mytilids the ligament is greatly extended along the dorsal margin and this portion termed the ligament margin can he differentiated from the remaining dorsal edge by a change in angle or curvature.
In cemented bivalves orientation varies between families but in the oysters, the only group represented in our fauna, they are always attached by the left valve. In many species the valves are not the same size or may not be the same shape. This, inequivalve , condition may be extreme as in Corbula or subtle as in tellins. The ligament joins the valves along the hinge margin, it is elastic, mostly brown or black in colour and serves to open the shell when the adductor muscles relax.
It is an important diagnostic character but is a complex structure and often difficult to interpret. The ligament nearly always lies between or posterior to the beaks. Some ligaments have both external and internal components. Most often used for the attachment pit of internal ligaments. Can be seen as a shallow triangular pit in the cardinal areas of Limopsis and Lima.
Typical of Mya and Scrobicularia. Often lost in disarticulated valves, typical of Lyonsia and Cuspidaria. Typical of venerids and tellinids. In those forms with teeth, their form, number and disposition are frequently of systematic significance.
There are four basic forms of hinge. Most bivalves exhibit scars on the interior of the valves that result from the attachment of muscles. In groups that become sessile and attached by a byssus the anterior portion of the shell is often reduced, resulting in the diminution of the anterior adductor.
This scar is situated close to the posterior adductor scar. The relative size and positioning of pedal and bysul muscle scars can be very important in separating closely related species of mytilids, giant dams and anomids.
The mantle tissue, which secretes the shell, is attached by a series of muscles close to the margins of the shell. The description of he pallial sinus is important. The dorsal section may be ascending directed towards umbo or descending towards ventral margin. The anterior section may be rounded sinus will be oval , straight sinus will be rectangular , or the dorsal section may meet the ventral section directly sinus will be acute.
For the ventral section other than the point of confluence, it is also important to note the angle at which it meets the pallial line. Present only in tellins but often obscure. To distinguish right from left valves requires discerning the anterior and posterior margins.
Once established if the shell is held with the hinge margin uppermost and the posterior and anterior in the same plane as the obrever then the right and left valves will be in the right and left hands of the observer. In shells with a external or sunken parivincular eg.
In shells with a pallial sinus this is always posterior, this is especially useful in taxa with internal ligaments. In shells witht an alivincular or duplivincular amphidetic ligament eg. In limopsids the posterior adductor is larger than the anterior, in glycymerids the converse applies. In general the mouth, labial palps and tip of the foot are anterior whereas the anus, rectum and siphons are posterior.
Orientation in nuculids and many galeommatids is difficult, many being slightly extended anteriorly, reference to the anatomy is the most reliable. The outline of the valves is variable and often subtle. Unlike leaf shape in botany the many forms have no absolute definition. Shell sculpture is a widely used character for identification of species but requires careful observation and description.
Numerical definitions can also be variable with numbers of a feature often increasing with growth. The following definitions give a guide to the terminology used here but may not agree exactly with others.
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