WHEN we speak of Crustacea, we think at once of the crabs and crab-like animals which are really the highest and largest members of the sub-group. There are, however, a host of small forms which are quite unlike the crabs both in structure and habit. Most of the newly hatched young are quite different from their parents and only attain to the parent form after a series of immature stages—I.e. the life-history of most Crustacea includes a complicated metamorphosis. The Crustacea group can be divided into two parts : one which contains all the familiar forms like the crabs, lobsters and shrimps, and a second, including all the other forms. Anyone
who has collected in ponds knows the water flea, the little Cypris, which looks like a bivalved shell-fish, and the Cyclops; a microscope must be employed to see the details of structure of these tiny creatures.
Water fleas are the most abundant of the fresh water Crustacea. They live in a sort of transparent bivalve shell from which their heads project. The popular name of water flea was given to it on account of its jumping actions when swimming. If you watch one under the microscope, you can see its method of swimming. It is carried through the water by backward strokes of its two antenna. Four to six pairs of legs act as paddles to create a current of water between the valves of the shell from back to front, and this carries small food bodies towards the mouth. The head bears a large eye. Between the shell and the animal’s back there is a space, which in the female is used as a brood-pouch; this is usually filled with developing but unfertilised eggs, for males are never found during the greater part of the year. These eggs give rise to tiny water fleas, so that there is no metamorphosis.
When the males appear, as they do at certain seasons, the females lay eggs which cannot develop without fertilisation, and if these eggs are examined, they are seen to be enclosed in a protective case which shelters them from the adverse conditions of the outer world. In this condition they can be transported either by the wind or by birds from one pond to another without injury. Not infrequently, water fleas suddenly appear in ornamental garden pools and are no doubt carried there by one of these agencies.
Cyclops is another common denizen of ponds, and its life-history includes a metamorphosis. Cyclops has a pear-shaped body which gradually narrows down behind to a forked tail. The head carries a single eye of a reddish colour. Unlike the water fleas, Cyclops has two pairs of antenna;, the second being much shorter than the first pair. It has mouth-parts which are jaw-feet, and four pairs of forked legs used in swimming. In contrast to the water fleas, Cyclops pass smoothly through the water. Females carrying long egg sacs can frequendy be collected from ponds. The eggs, however, do not give rise to small replicas ol the adult, but hatch out into little oval larva; (an immature stage which is unlike the adult) with three pairs of limbs and a single eye. This larva is called a iiaapliiis and it eventually develops into the adult.
The relatives of Cyclops are numerous and diverse. Many of them are marine, living near the surface of the sea. Some are economically important as food for herring; others, such as fish lice, are parasitic. Some, through their parasitic habit, degenerate into immobile, shapeless masses. Others are numbered amongst the most beautiful creatures of tropical seas.
AN ANIMAL THAT KICKS FOOD INTO ITS MOUTH ANOTHER well-known creature in this group is the barnacle, so common above low-water mark on the rocks of our coasts. Older naturalists thought barnacles were shell-fish, which is not surprising; later, it was seen that the eggs gave rise to a nauplius larva which linked them up with other crustacea. This nauplius larva swims about for a time and then changes into another larval form which resembles the bivalved cypris already mentioned and is therefore called the Cypris larva. It possesses six pairs of legs, and the antenna: carry suckers with which the animal eventually attaches itself to a rock, when the shell falls off and development proceeds. Huxley described the barnacle as ‘a Crustacean fixed by its head and kicking the food into its mouth with its legs.’ If an acorn barnacle is watched, these feet will be seen at work sweeping the surrounding water for prey. Little valves of the adult shell open and close to allow the legs to protrude.
A relative of the barnacle is the parasite Sacculina, which is frequently found under the abdomens of crabs. It sends rootlike processes through the crab’s armour, and these eventually penetrate to every organ of the body and absorb nourishment. Sometimes the presence of the parasite changes the sex of the crab. Through parasitism, great degeneration has taken place and there is not the slightest hint that the adult Sacculina is an Arthropod at all—but the life-history gives it away and we find the characteristic stages of a crustacean’s life.