258 THE POPULAR SCIENCE MONTHLY 



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 antennae 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 earth- 

 worm. 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 ccelenterates as motor cells, for though some of them undoubt- 

 edly 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 inter- 

 vention 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, oesophageal connectives, and a ventral, seg- 

 mented cord. These organs have been formed by a process of delami- 

 nation as in the earthworm and exhibit the same fundamental arrange- 



