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

BOOK 2

theless, the barnacle begins its life as a freeswimming larva which is obviously a crustacean, and later settles down and changes its shape to suit its new sedentary life. It does, in fact, very much what the sea-squirt does. In a parasitic animal, Sacculina, which is classified with the barnacles, the change is even more profound. The newly hatched Sacculina is a free-living creature with outside skeleton, limbs, eyes, heart, and so forth; but, a sign of its destiny, it has no digestive tube. It moults and acquires a new form, in which it resembles one of the water-fleas, Cypris, much as an embryo man resembles a fish, and it then swims around looking for a crab. Having found a suitable victim, it settles down somewhere on his body, loses its structural specialization to become a mere mass of hungry cells, and burrows through his armour into his blood-stream. By the host’s blood the parasite is nourished, and it rolls hither and thither for a time, moved by the currents of blood in the body-cavity. Finally it settles down in the back part of the cephalothorax, just under the abdomen, and becomes a mass of parasitic tissue (Fig. 82) ; it sends greedy roots into every corner of the crab’s body (excepting only his heart and his nervous system, for no provident parasite kills its host) and sucks nourishment from his tissues as a plant sucks moisture from the soil. Thus all its chemical needs are satisfied, and the only care left is reproduction ; it becomes a mere mass of ova and spermatozoa (for it is hermaphrodite) with a halo of absorbing roots, and ends its career by fertilizing its own eggs.

Sacculina is by no means the only parasite among the crustacea. Altogether some 700 parasitic species are known—and these are only a small proportion of an enormous and varied class.

§ 3b Spiders, Mites, and Scorpions

The second class of arthropods, the Arachnida, includes a great number of landliving animals. ‘The better known kindssuch as the spiders and ticks—may be distinguished from other land-living arthropods by their possession of four pairs of walking legs.

Many of the land arachnids possess a curious method of breathing, found only in this class, in the insects and the centipedes, and in that curious little animal, Peripatus. The body is permeated by a branching network of air-tubes or trachee, stiffened by

126

THE SCIENCE OF LIFE

CHAPTER 2

rings and spirals of chitin and opening to the exterior at a number of places on the bodywall. Air is pumped in and out of the trachee by means of rhythmical movements of the body—movements that can be seen very clearly in the abdomen of a wasp—and it is carried by the fine end-branches of the tubes directly to the tissues themselves. There is no need with a system of this kind for the blood to transport oxygen or carbon dioxide ; the trachee are a device for putting all the tissues of the body in immediate contact with fresh air. But some arachnids have, instead of trachez, a lung-like structure in which the blood is oxygenated, and a few have both mechanisms together.

The carnivorous spiders are the bestknown members of the class. They include, besides the cruel, skilful architects with whose snares we are familiar, a number of prowling and hunting forms which capture their prey by stalking or chasing it, and which have no need of webs. The phalangids or harvest-men, little round bodies moving on eight long, stilt-like legs, are put in a separate order from the spiders because their body is not divided in two by a waist, like that ofa spider. To the arachnids belong also the scorpions and one or two small and comparatively unimportant orders. But the order which is most important to man is the great army of mites and ticks, some biting us, sucking our blood, burrowing into our flesh and infecting us with various diseases, some attacking our cattle, some producing galls in our crops and forests, some living innocently, as far as we are concerned, in ponds and ditches, blasted trees, old cheese, and all sorts of other places.

The class also includes that curious marine creature the king-crab, covered by an enormous circular carapace, and suggestive of a small boy crawling about under a tin bath with a broomstick trailing behind. And there is an extinct group of gigantic marine arachnids, the sea-scorpions, some of which were as much as eight feet long. In the remote geological past, before even the appearance of the earliest vertebrate, the sea-scorpions were the bullies of their world, the strongest and most impressive living things.

§ 3c Insects

The Insecta are an enormous group; in the rich variety of their forms they surpass every other class in the Animal Kingdomindeed, there are more species of insects