Why do bivalves have small heads

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:. Bivalve molluscs are completely enclosed by a shell made of two valves hinged at the top. A hinge ligament made of elastic protein joins the two halves of the shell together, and large adductor muscles between the two valves hold them closed. When the shell is closed, the top part of the hinge ligament is stretched and the lower part is compressed.

This means that when the adductor muscles relax, the ligament acts like a spring and the shell opens. The hinge ligament is usually external to the shell, but in many species has shifted to an internal position between the hinge teeth. The hinge teeth consisting of cardinal and lateral teeth interlock on the two valves preventing slippage. Differences in the nature of the hinge teeth are important in the classification of bivalves.

Many bivalve species, in particular oysters, have been cultured for many years. Aquaculture is one of the main reasons why there are so many introduced species.

Either a few specimens escape from the culture plots during transfer of spat or during harvesting and establish wild populations, or planktonic larvae from cultured specimens disperse to neighbouring areas. Transport of live oysters across borders and continents can also move parasites and epifauna and —flora. Also, transport of "seed" oysters or mussels, sometimes called "spat", can be responsible for introduction of associated fauna and flora.

Inside of Mya arenia shell showing adductor muscle scars and pallial sinus. Photo by Kathe Rose Jensen. Almost all bivalves are suspension feeders. They pump water through the inhalent siphon , across the large, complexly folded gills , where food particles, mostly phytoplankton suspended in the water column, is filtered out of the water, which is then pumped out through the exhalent siphon.

These rejected particles are covered in mucus and expelled as so-called "pseudo-faeces". Bivalves are able to pump large volumes of water and hence to clear the water column of organic particles. On the other hand, the accumulation of faecal particles and pseudo-faeces increases the particulate organic matter in the surface sediment, which may increase oxygen consumption. This is the case in the coal-bearing rocks of the Late Carboniferous in which Carbonicola has been used to identify and correlate individual coal seams.

The strata of the Late Cretaceous Chalk Group, forming the famous White Cliffs of Dover, are rich in the remains of inoceramid bivalves. They have been used to refine the established Chalk biostratigraphy and so aid correlation.

The strong influence of environmental factors on bivalve distribution coupled with rather slow rates of evolution limits their widespread use for biostratigraphy.

Even in the rather uniform environment of the Chalk, some groups display faunal provincialism. Some 17th and 18th century Scottish accounts show that its possession was believed to cure arthritis.

In the old Scunthorpe coat of arms, images of this oyster signify its occurrence in the formerly commercially important Jurassic ironstone deposits of the area.

Courtesy of the North Lincolnshire Council. Illustrated by Robert Plot. Many of the fossils in the BGS palaeontology collections are available to view and download as 3D models. Discovering Geology introduces a range of geoscience topics to school-age students and learners of all ages. Take a look at the history of the Earth, from its formation over four and a half billion years ago to present times. Home » Discovering Geology » Fossils and geological time » Bivalves.

Bivalves Discovering Geology — Fossils and geological time. Laevitrigonia gibbosa J Sowerby, has valves that are mirror images. Shape Bivalve shells may be elongate, round or highly irregular in shape. Crystal layers The valves consist of layers of crystals of the minerals calcite or aragonite.