The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

are active beasts of prey. ‘The leeches differ from the other two groups in a number of important respects, so that it is customary to divide the annelids into two classes, one comprising the earthworms and the more varied marine group and the other the leeches.

85 Roundworms

The phylum Nemathelminthes or roundworms includes a number of worms of plain, unassuming appearance but nevertheless of great importance to ourselves because a number of them are parasitic. The roundworms often found in the intestines of pigs, horses, and men are typical of the group. They have thin cylindrical bodies, several inches long, pale buff in colour, and tapering away at both ends. Their anatomy shows well the simplicity of structure that is often found in parasitic organisms. Living surrounded by digested food, all they have to do is to gulp it in to their own bowels and absorb it; they have no need of glands to manufacture digestive juices, and their

alimentary apparatus consists of two parts only—a muscular pump, the pharynx, which sucks the juices in, and a simple intestine

BR. Brick —

Fig. go.

TEE SGLEN CE OF nln

A group of Marine Worms

CHAPTER 3

where they are absorbed. Moreover, as they inhabit a warm, sheltered place and have neither prey to seek nor enemies to fear, their behaviour is almost non-existent. ‘They have no eyes or other specialized senseorgans—indeed, eyes would be useless in the darkness of our bellies—and their nervous and muscular systems are very elementary in structure. They can perform simple writhing movements, but it may be presumed that they do not do so very often; beyond reproducing themselves and occasionally dodging an exceptionally violent peristaltic wave it is hard to see what movements they require. Correlated with this low level of muscular activity we find that they possess no special circulatory or respiratory organs ; their muscles do so little work that all the oxygen they need can diffuse in from outside.

It is at first sight difficult to explain how an animal which spends its life in an intestine can get any oxygen at all, and indeed it used to be thought that the parasitic roundworms depended upon a different kind of chemical mechanism for their vital energy. But the story has now been made clear. When the host-animal is not digesting a meal there is very little oxygen, if any, in his bowels ; under these conditions the round-

Below, left to right, are two iridescent Sea-mice (Aphrodite), a burrowing Lug-worm (Arenicola) with its tufted gills, and

a Scale-worm (Lepidonotus).

Above are a bronze-coloured Rag-worm (Nereis), and two different kinds of tube-building

worms (Sabella and Amphitrite), spreading their tentacles for oxygen and food.

138