Fig. 51.—Molluscs—Cuttle-fish, with eggs.
Our third great group is that of the Arthropods (literally 'jointed footed'), including the Crustaceans (crabs, lobsters, shrimps, etc.), spiders and mites, centipedes and insects. The Arthropods are sometimes classed together with their ancestors, the ringed worms (such as the common earth-worm), as Articulata, a name which refers to a very obvious feature, the repetition of similar segments in a regular series from front to rear. This is perhaps most apparent in the ringed worms and centipedes, but it is to be seen in all members of the group. This same tendency to reduplication of parts in a regular series may be observed in the vertebrates, as we shall see. Slight indications of it are also to be found in the Nemertines. Numerous theories have been proposed which derive the vertebrates from some of the Articulata—from the ringed worms or the Crustaceans, and even from the air-breathing members; and at first sight such theories seem attractive, for in some of their more obvious characters there is a certain resemblance between the two groups. But there are also many and fundamental differences, and few zoologists have accepted any hypothesis of this type. We may briefly allude to some of these differences.
Fig. 52.
1, Marine swimming ringed worm; 2, giant centipede; 3, Peripatus.
Photo: Martin Duncan, Berridge, and Bastin.
In the Arthropods, where the body consists of hard and soft parts, the 'skeleton' is an external one, and encloses the soft parts. Respiration occurs by means of the skin or of gills, or, in air-breathing forms, by 'trachea,' which are small branching tubes opening on the sides of the body. But in no case has the mouth or the digestive tract any connection with the respiratory system, a condition of affairs very different from that obtaining in the vertebrates. The nervous system consists of a brain, situated above the gullet, a nerve ring round the latter, and a double nerve cord running along the body, below the digestive canal. This is obviously the opposite position to that occupied by the main nerve cord in the vertebrates, an important point of difference.
The Arthropods are an extraordinarily successful group. A multitude of forms of Crustaceans populate the waters, and they are excelled in numbers and variety only by the insects upon land. While the individual size appears to be somewhat strictly limited, probably by the nature of the respiratory and blood systems, many types show exceedingly high development in various directions—in intelligence, in social and parental instincts, etc. The insects are of course to be regarded as the highest Articulata, and have, like the highest vertebrates, the mammals and birds, almost completely forsaken the water for the dry land and the air. An interesting member of the Articulata, from the standpoint of the Evolution theory, is the Peripatus, shown with a ringed worm on Fig. 52 (3). It gains its interest for us from the fact that, while classed as an Arthropod, it stands very nearly half-way between the ringed worms (Fig. 52 (1)) and the true Arthropoda, and thus forms a solitary link between the two types. In Fig. 52 (2) and Figs. 53 to 58 are shown a number of types of Arthropods.
Fig. 53.—Arthropods—The Lobster.