Page:Popular Science Monthly Volume 75.djvu/143

Rh from the sense-cells, numerous multipolar ganglion-cells whose processes add to the fibrillar material already mentioned. A careful study of this fibrillar material has recently been made with the result that a true nervous network has been demonstrated in hydroids (Wolff, 1904; Hadzi, 1909), siphonophores (Schaeppi, 1904) and sea-anemones (Wolff, 1904; Groselj, 1909). In the sea-anemones in particular this network appears to be a perfectly continuous and diffuse one, notwithstanding Havetfs previous declaration (1901) to the contrary. The third layer is composed of parallel muscle-fibers that rest against the supporting lamella on one side and are in contact with the nervous network on the other side. The muscle-cells of this layer are much elongated, spindle-shaped cells. These three layers, the epithelial layer, the nervous layer and the muscular layer, constitute the structural elements in the ectodermic neuromuscular mechanism of a sea-anemone.

The nervous type of ectoderm just described covers practically the whole surface of a sea-anemone and has been designated as a diffuse nervous system in contrast to a centralized one. The fact that the nervous layer is more fully developed on the oral disk than elsewhere has given anatomical grounds for the assumption that this portion is a central nervous organ, but, as will be shown later, the physiological evidence in favor of this opinion is so slight that the designation of the nervous system as a diffuse one is more consistent with facts.

From the standpoint of our original analysis, it is quite plain that in the sea-anemones we are dealing with at least two elements of the typical neuromuscular mechanism, namely, receptors as represented by the sense-cells, and effectors as seen in the muscle-fibers. Whether the fibrillar material that intervenes between these two structures represents an adjustor or central apparatus will be discussed after the action of this nervous mechanism has been more fully described.

The feeding habits of the sea-anemones throw considerable light on the physiology of their nervous structures. If particles of meat are dropped on the tentacles of an expanded Metridium, they become entangled in the mucus on these organs and are quickly delivered to the mouth, where they are swallowed. If fragments of clean filter-paper soaked in sea water are similarly dropped on the tentacles, they are usually discharged from the edge of the oral disk without having been brought to the mouth. Thus the animal appears to discriminate between what is good for food and what is not. If, however, pieces of filter-paper soaked with meat juice are put on the tentacles, they are usually swallowed as though the sea-anemone had been deceived. On the basis of these simple experiments a still more striking combination can be devised. If a sea-anemone is provided alternately with pieces of meat and pieces of filter-paper soaked in meat juice it will in the beginning swallow in sequence both materials, but after ten or a dozen