Page:Popular Science Monthly Volume 75.djvu/262

258 more extensive organ connected with a considerable number of sense organs none of which are present in the earthworm. Eight peristomial tentacles, a pair of palps, a pair of antennæ and, two pairs of eyes are found connected by nerves with the brain of Nereis and represent a condition in strong contrast with the unspecialized state in the earthworm. Yet both the earthworm and Nereis show much the same traits when deprived of their brains (Loeb, 1894). Each worm is immensely reduced in activity somewhat as a jellyfish is after the removal of its sense-bodies, and one is justified in concluding that the head of even the earthworm is an especially sensitive region through which many slight environmental influences that might not be able to affect other parts of the body gain access at this point to the neuromuscular mechanism. That such a condition should obtain at the anterior end of a bilateral animal has long been recognized as appropriate, for this is the part of the animal that in normal locomotion first reaches the new environment. But I am not acquainted with any discussion as to the mutual relations of the nervous parts at the anterior end of an animal so far as their origins are concerned. If what has been said in these lectures is true, namely, that sense-organs in general precede central nervous organs in evolution, then the brain of the worm has developed at its anterior end because the chief sense-organs were originally there, and not vice versa, a statement that I believe to hold for the growth of the brain in all animals. Intricate and marvelous as the brain of the higher animals is, it is, in my opinion, the product of a group of sense-organs that in evolution preceded it in point of time.

The annelids then possess a neuromuscular mechanism in which there are not only primary organs such as muscles, and secondary organs, the sense-organs, but also tertiary organs, central nervous organs. These central organs intervene in position between the receptors and effectors and in the annelids are composed almost exclusively of short overlapping neurones. It is probable that in the sea-anemone these neurones are represented by the so-called ganglion-cells of the nervous layer, but I would not go as far as Havet (1901) and designate these cells in cœlenterates as motor cells, for though some of them undoubtedly connect with the muscle-fibers, others may be purely association neurones. I believe further that in the sea-anemones the fibrils from many sense-cells connect directly with muscle-fibers without the intervention of ganglion-cells.

As an example of a central nervous system built upon the annelid type but with increased complication, we may turn to the arthropods. The central nervous system of these animals, like that of the annelids, consists of a dorsal brain, œsophageal connectives, and a ventral, segmented cord. These organs have been formed by a process of delamination as in the earthworm and exhibit the same fundamental