There is one very obvious characteristic of molluscs that the platyhelminthes, nemetenes and other worms do not share, and that is the dorsal shell. The shell, made of protein and reinforced by calcareous spicules or from one to eight calcareous plates, secreted by the dorsal and lateral epidermis, is in some species of mollusc very much reduced and covered with tissue, or lost completely, but in many it remains as the protective covering that may have been its original purpose. For flatworms in the sea, a shell would not have been a particularly unlikely thing to evolve. Many flatworms defend themselves by quickly burrowing down into the mud on detection of a predator, as the soft bodies of a worm provide no defence against a larger species searching nutrition. In the sea, calcium is highly abundant, and to accumulate calcium on the body surface would provide some, if at first limited, protection for the worm that reduced the need to bury in the ground as soon as a predator came near. The first step in the evolution of a shell may have been to thicken the endoderm and produce calcium spines in it. The more calcium spines that are produced, the more likely they are to be able to fuse together and form a shell or a series of shell plates. This provided very good protection, especially as they were the first animals to develop hard structure and no other animals could eat them as they had not yet developed teeth.
However, the problem arising from this hard shell may have been the stimulus for the evolution of the next main adaptation of molluscs from worms. The shell is non-permeable, and gas exchange through the body becomes impossible with the addition of this structure. The molluscs were the first animals to leave a clear fossil record because of their hard shells, and this helps us to see how they adapted themselves for breathing, as we can see how the shell rose over time. Molluscs formed gills to breathe once the body surface area was no longer useful. They began on the outside surface of the shell in some species, and in others ctenidia were housed primitively in a posterior mantle cavity concealed beneath the shell. However, the rest of the body was so well protected form predation that the exposed gills on the shell surface really disadvantaged the animals, and gradually they moved down to the bottom of the shell and onto the animal. The shell became raised to make room for the gills underneath. We can see that the shells keeps rising throughout the fossil records, providing more room for both gills and gonads. There is plenty of carbon available in the sea for the molluscs to make use of, and the support of the sea makes it easy to carry a relatively large shell. However, there is an upper limit, as beyond a certain size the shell can get caught up in strong currents and push the animal over so it cannot get up. This meant that the shell could not get beyond a certain size, until a mutation changed the direction of growth, making the shell spiral, like the snail we now see.
As the sea-snails moved onto land, the shell size had to be drastically reduced as the animal could not carry the shell without the supporting weight of the water. The land molluscs have very thin shells. Apart from that, it was not too difficult for the molluscs to move from the sea onto the land, as they are one of the very few groups of animals to possess the enzymes capable of breaking down the refractory structural carbohydrates which characterise the larger sea-weeds and the land plants.
Another specialisation that removes the molluscs from the flatworms that they are similar to but not in the same group as, is the radula. This is a toothed, tongue-like organ in the buccal cavity, which would have been used in the sea to rasp algal films and colonies from hard surfaces such as the rocks they frequently live on, and it is widely believed that molluscs were in fact shallow-water grazers of algal protests in their early stages of differentiation.
There are, of course, many features of modern molluscs that have developed to a point where they bear little resemblance to the flatworms they descended from. The chaetodermomorpha are relatively ‘worm-like’ – They have cylindrical bodies, elongated along the anteroposterior axis, and movement is achieved by peristaltic action along the length of the body. Some species lack a radula, and instead of a shell, the epidermis secretes a chitinous cuticle. The neomeniomorpha are also shell-less and vermiformly elongated along the anteroposterior axis, but are more closely related to the shell-bearing groups, as the body is laterally compressed, and has a ventral groove representing a highly reduced foot. They lack ctenidia, but often another form of more primitive gills develops, and the body is covered (apart from the primitive foot) with a mantle, embedded underneath with small calcareous scales or spicules.
The monoplacophora are deposit feeders with a small cap-shaped shell, a weakly muscular circular foot, and multiple pairs of many organs. The polyplacophera are distinguished by their eight, overlapping dorsal shell-plates enabling them to roll up into a ball. They have a large muscular foot that can display suction, and mainly feed on algae using the well-developed radula.
The gastropoda are a very large and diverse group, with a rotated visceral hump, so that the mantle cavity faces forwards but the anus and kidneys discharge anteriorly. They have a well-developed crawling foot, and a well-defined head bearing a radula, a pair of jaws, a pair of eyes and one or more pairs of sensory tentacles. In total, the gastropoda includes 77,000species of slug- or snail-like molluscs, distributed between 23 orders. The class bivalvia are essentially laterally compressed molluscs completely enclosed within a pair of shell valves. They are relatively sedentary, and the head is greatly reduced, without the radula, eyes and tentacles. The two sides of the shell open passively with an elastic dorsal ligament, but must be kept closed actively, using two adductor muscles. The relaxation of the adductor muscles allows a water current to be pulled through the cavity, and species that burrow into soft sediment have their foot slightly sticking out of their shell. In the primitive forms, the ctenidia serve mainly the ancestral gas exchange function, and feeding is achieved by means of labial palps situated either side of the mouth. However, in the majority of species the ctenidia ate folded to form the organs of filter feeding.
The scaphopoda, or tusk-shells, are elongate, cylindrical molluscs almost entirely enclosed by the mantle, which secretes a single, tubular, calcareous shell, open at each end. They lack eyes and sensory tentacles, but have a radula, a single median jaw, and paired clusters of narrowed, clubbed contractile filaments called the captacula, used for deposit feeding. The cephalopods are the most sophisticated class of molluscs. The anterior region of the embryonic foot develops into a series of prehensile arms or tentacles around the mouth, and the posterior portion forms a muscular funnel around part of the opening of the mantle cavity, which can take in and forcibly expel water to provide movement for the animal. They, like the gastropods, are elongated in the dorsoventral plane. Prey is caught by the arms, but many cephalopods are large animals catching large prey that have to be macerated by a pair of beak-like jaws before worked on by the radula. The nautiloidea have a single, external planospiral shell, only the last chamber of which is occupied. The coleoidea however show a marked reduction in shell (which is often lost) and are much more proficient swimmers. They also possess a closed blood system and a highly concentrated nervous system, which includes well-developed eyes.
Despite the diversity I have just discussed, it is from roughly the animal described in the first few paragraphs of this essay, adapted from the flatworm but with the addition of a shell, gills, and improved feeding apparatus, which most modern molluscs developed from. There are currently many diverse species of mollusc, but all share characteristics that link them back to this animal – a specialised worm.
