In a very few simple bivalves (Protobranchs) the gills remain confined to a cavity at the rear of the body and the filaments are short. In more advanced forms each filament becomes elongated and turns upwards. The gills are also extended so that they run practically the length of the body, Water is drawn into the shell at the rear end where the mantle is often extended backwards to form two siphons. The lower siphon takes the water in. The water provides not only oxygen. Particles of food in the water are swept forward and downward, moved along the, bottom of the gill filaments by the rapid beating of tracts of cilia. During the passage forward the particles become entangled in mucus.

From the gills, food particles are swept onto the labial paips about the mouth. The labial palps are also ciliated. Material may be swept into the mouth or rejected and passed back by ciliary action along the base of the gills to the washed out through the upper of the two siphons. Sorting at the labial palps selects the finest particles, not necessarily the most nutritious. Large, coarse particles are rejected for bivalves have not masticating radula to break up food. Food particles are swept down a short gullet to the stomach. Cilia in the stomach further sort the particles and useless material is swept straight into the intestine.

Some preliminary digestion does take place in the stomach. Digestive enzymes, for breaking down carbohydrates, are secreted there by a very strange method. Inside a diverticulum of the stomach is embedded a tough, rod like structure called a crystalline style which contains the digestive enzymes. The style gradually dissolves, releasing the enzymes but cells lining the diverticulum continually secrete new material behind. The style rotates and acts as a stirring rod and also as a windlass; threads of mucus bearing food are drawn from mouth into stomach.

Minute food particles are next taken via a pair of ducts to the digestive gland whose cells actually absorb the particles and digestion is completed inside them. Bivalve molluscs, like snails, retain a muscular foot lying between the two sets of gills on the ventral side of the body. Its shape and position is altered by muscular contraction and can be protruded through the gape of the shell.

In one group of bivalves the so called Normal bivalves the foot is well developed and the creatures use it to move about at the surface (e.g. cockles) or in some instances to burrow (e.g. razor shells). The giant clams of the Pacific and Indian Oceans belong to this group. They may grow to be more than a yard in length.

The Sessile group of bivalves have, in general, lost their mobility in the adult animal Instead, they have become firmly attached to the surface. Modifications have accompanied this stationary mode of life. The ventral foot has tended to move more and more towards the front of the animal. As a direct result the anterior adductor muscle has become small or is lost; usually each valve becomes highly asymmetrical in shape.

A byssus nearly always appears at some stage in the life history. A byssus is a mass of sticky, diverging threads arising from a pit at the back of the foot. In the mussel the byssus protrudes through the gape in the valves. The pearl oyster has come to lie on its right valve and the byssus emerges through a notch.

The common oysters have lost the foot and byssus altogether; the right valve becomes cemented to the ground.
Pecten. the scallop, is structurally a sessile bivalve. The front adductor muscle has been lost by the forward migration of the foot. But the remaining muscale has become enlarged. By rapid contraction the valves are clapped together and water is violently discharged. This is an efficient. mechanism for swimming and Pecten has given up its sessile habit.

The last division of the bivalves is the deep burrowers. Unlike the burrowers of the Normal group, the deep burrowers, have become extremely modified to their mode of life. The valves are only weakly hinged; often they have completely lost their ligament and the valves permanently gap . The siphons are enormous, often dwarfing the ;rest of the body.

Lamp Shells - (See Brachiopoda).

Larva - Young form of an animal that is very different from the adult.

Examples include caterpillar adult. Examples include caterpillar (adult is butterfly), tadpole (frog), and the various crustacean larvae. Some animals pass through more than one larval form before reaching maturity.

Larynx - Expanded upper region of the windpipe the voice box.

Lateral Line System - A remarkable system of sens organs found only in fishes and in the young stages of amphibians. It is better developed in bony fishes than in shark-like fishes. The lateral line can clearly be seen running along the sides of both sharks and bony fishes from the back of the head to the tail. A pattern of similar lines can be seen on the head. They consist of distinct groups of sensory cells arranged in long rows and supplied with nerve endings.

The cells are protected either in an open groove running just beneath the surface of the skin or in a tube which opens to the exterior (through the scales in bony fishes, between them in sharks) at intervals by a series of pores. Each cell has a hair like process projecting into the water. in the groove or tube. Movements and vibrations in the water round the fish move the hairs and the disturbance of the sense cell results in signals passing along nerve fibres to the brain.

The lines on the head are particularly well developed in the herring and other plankton-feeding fish. The system may play an important part in the detection of food. A certain deep water fish has lateral line organs on stalks projecting from the sides of the body. Food is scarce in the abyssal regions of the sea: is it possible that this extraordinary development of the lateral line system helps the fish to detect prey? Alternatively, of course, it would be equally valuable in detecting disturbances of the water due to enemies.

Lepidotera - Order of Insecta, containing the butterflies and moths. The characteristic feature is the clothing of body and wings with tiny scales which are responsible for the many bright colours to be found in the order. The adults are typically nectar feeders, sucking up the liquid through a long tongue or proboscis which, when not in use, is coiled up underneath the head. Metamorphosis is complete, the larvae (caterpillars) have biting mouths and often possessing stumpy prolegs on the abdomen.

Leucocyte - Name given to the various white blood cells.

Life Cycle - The series of changes undergone by an organism during its life time from fertilisation to death. In most animals this is a simple cycle but where there is alternation, the cycle includes both forms and does not end with the death of anyone the them.

Ligament - Tough strip of collagen fibres that joins bones together at joint. Also the horny hinge of a bivalve shell

Linkage - The association of genes, on one chromosome so that the features they produce in the organism will always tend to appear together. This is because complete chromosomes are normally passed on to the next generation and if ones gene goes, so will those linked to it.

Littoral - Inhabiting the shores and surrounding regions of the sea bed.