Some members of the sea urchin family have hoof-like formations on the ends of some of their spines, with which they are enabled to walk over the sea bottom without using the suction disks. About the only enemy of these fearsome nightmares of the deep is man. Some species are used extensively for human food, notably among the Mediterranean coast and in the West Indies. The developing eggs are taken from the body and eaten either raw or cooked. Even if it should prove suitable for human food, it is unlikely that the sea porcupine ever will be a rival in this respect of its rival, the “sea rabbit.” It is too secluded in its habitat.
Worms That Are Unkillable
In nematodes life may have reached its greatest capacity for survival. The remarkable persistence of these soil worms has been studied by C. W. McBeth, researcher of the Shell Oil Company. One form, he reports, has been known to survive after 25 years in a glass bottle in a laboratory. Another, a pest of wheat kernels, apparently came back to life after 28 years in laboratory storage. A nematode which had invaded a rye plant, collected in Kansas in 1906, revived after 39 years of complete dehydration in a herbarium.
Those which live as active feeders in the soil, however, are not particularly long-lived. Each species depends on a certain plant type and must starve if this is not available. The recently introduced golden nematode of potatoes, a particularly obnoxious pest, is known, however, to survive as much as ten years in soils where no potatoes are planted. A great mass of eggs is produced, but not laid. They are retained in the body of the mother, who dies. Her skin remains—a bag filled with eggs.
This stays in the soil, apparently unharmed by changing conditions, until potatoes are planted again. Then some mysterious influence, as yet unexplained, causes the eggs to hatch and the whole nematode cycle begins once more.
Due to such a strange tenacity of life this nematode is about the hardest of pests to control. It refuses to stay dead. Other species likewise are specialized in one or more ways of survival under adverse conditions.
Because of the complexity and minuteness of the nematodes, it has been very difficult to determine the effects of heat, cold, flooding and drying on different species. These vary for each. One nematode species, especially resistant to drying, has a skin consisting of nine layers. The ability of this skin to hold moisture inside the minute body undoubtedly is an important defense mechanism. Some species are entirely marine, others are parasites within the bodies of other animals. It has been found that both of these varieties possess skins which are much more permeable to moisture. The original home of the phylum probably was in the sea, but a moisture-proof cuticle has been developed by those which have invaded the land.
The whole body structure of the plant nematode is almost ideally suited to life in the soil. The typical eel-shaped body is well-adapted for moving in the moisture surrounding soil particles. Deviations from this eel-form in certain stages of some species, usually in mature females, are found only in sedentary stages. The larvae and males retain the ancestral shapes. Another deviation is found in the so-called “ring nematodes” which have short, plump bodies incapable of locomotion in the typical whip-like fashion. Movement is accomplished by alternate expansion and contraction of the body.
A majority of nematodes spend a greater part of their lives in the soil. A few, however, are carried from plant to plant by insects. Although moisture is necessary if the tiny animals are to remain active, the soil seldom becomes too dry for them except in the top two or three inches. Their structure is well-adapted for moving up and down.