The articulate series consists of two groups of animals. The higher group includes the crabs, spiders, thousand-legs, and finally the insects, and forms the kingdom of arthropoda. The lower members are still usually reckoned as worms, and are included under the annelids. Of these our common earthworm is a good example, and near them belong the leeches. But the marine annelids, of which nereis, or a clam-worm, is a good example, are more typical. They are often quite large, a foot or even more in length. They are composed of many, often several hundred, rings or segments. Between these the body-wall is thin, so that the segments move easily upon each other, and thus the animal can creep or writhe.

7. EUNICE LIMOSA (ANNELID). LANG, FROM EHLERS.

Front and hind end seen from dorsal surface. fa, fp, fc, feelers; a, eye; k, gill; p, parapodia; ac, anal cirri.

[LARGER]

These segments are very much alike except the first two and the last. If we examine one from the middle of the body we shall find its structure very much like that of our schematic worm. Outside we find a very thin, horny cuticle, secreted by the layer of cells just beneath it, the hypodermis. Beneath the skin we find a thin layer of transverse muscles, and then four heavy bands of longitudinal muscles. These latter have been grouped in the four quadrants, a much more effective arrangement than the cylindrical layer of the schematic worm. Furthermore, the animal has on each segment a pair of fin-like projections, stiffened with bristles, the parapodia. These are moved by special muscles and form effective organs of creeping.

Within the muscles is the perivisceral cavity, and in its central axis the intestine, segmented like the body-wall. The reproductive organs are formed from patches of the lining of the perivisceral cavity, and the reproductive elements, when fully developed, fall into the perivisceral fluid and are carried out by nephridia, just such as we found in the schematic worm. Beside the perivisceral cavity and its fluid there is a special circulatory system. This consists mainly of one long tube above the intestine and a second below, with often several smaller parallel tubes. Transverse vessels run from these to all parts of the body. The dorsal tube pulsates and thus acts as a heart. The surface of the body no longer suffices to gather oxygen, hence we find special feathery gills on the parapodia. But these gills are merely expanded portions of the body wall, arranged so as to offer the greatest possible amount of surface where the capillaries of the blood system can be almost immediately in contact with the surrounding water.

The nervous system consists of a large supra-œsophageal ganglion in the first segment; then of a chain of ganglia, one to each segment, on the ventral side of the body. With one ganglion in each segment there is far more controlling, perceptive, ganglionic material than in lower worms. Furthermore the supra-œsophageal ganglion is relieved of a large part of the direct control of the muscles of each segment, and is becoming more a centre of control and perception for the body as a whole. It is more like our brain, commander-in-chief, the other ganglia constituting its staff. The sense-organs have improved greatly. There are tentacles and otolith vesicles as very delicate organs of feeling, or possibly of hearing also.

But the annelids were probably the first animals to develop an eye capable of forming an image of external objects. The importance of this organ in the pursuit of food or the escape from enemies can scarcely be over-estimated. The lining of the mouth and pharynx can be protruded as a proboscis, and drawn back by powerful muscles, and is armed with two or more horny claws. Eyes and claws gave them a great advantage over their not quite blind but really visionless and comparatively defenceless neighbors, and they must have wrought terrible extinction of lower and older forms. But while we cannot over-estimate the importance of these eyes, we can easily exaggerate their perfectness. They were of short range, fitted for seeing objects only a few inches distant, and the image was very imperfect in detail. But the plan or fundamental scheme of these eyes is correct and capable of indefinitely greater development than the organs of touch or smell, perhaps greater even than the otolith vesicle.