NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 465 



more primitive one than that of the latter and nearer to the ancestral forms. 

 Parker's theory regards the muscle cell, or " effector," as developed from amoeboid 

 epithelium and as being gradually displaced to form a layer underneath the 

 external epithelium. " Next in sequence would appear the receptor or sense organ 

 which, derived from the cells in the neighbourhood of a developed effector (see 

 Fig. 141), would serve as a more efficient means (D) of calling this organ into action 

 than direct stimulation. This stage is represented by many Ccelenterates ; and 

 their quick responses, as compared with those of sponges, are dependent, I believe, 

 upon this advance in organisation. Finally, in forms somewhat more advanced 

 than the Ccelenterates, central nervous organs or adjusters would begin to 

 differentiate in the region between the receptors and effectors ; and these would 

 develop in the higher animals first, as organs of transmission whereby the whole 

 musculature of a given form could be brought into co-ordinated action from a 

 single point on its surface and, secondly, as the storehouse for the nervous 

 experience of the individual and the seat of those remarkable activities that we 

 recognise in the conscious states of the higher animals. Thus nerve and muscle 

 did not develop independently, as claimed by Glaus and Chun, or simultaneously, as 

 maintained by Kleinenberg and the Hertwigs, but muscle appeared first as inde- 

 pendent effectors and nerve developed secondarily in conjunction with such muscles, 

 first as a means of quickly setting them in action and, secondly, as a seat of 

 intelligence" (Parker, 1911, pp. 224-225). The same author further points out 

 (1909, p. 58) that a receptor or sense organ alone would be of no service to an 

 organism, neither would nerves nor nerve centres alone, whereas a muscle cell, or 

 effector, is of use, if it can be stimulated directly. It is thus not improbable that 

 there should be primitive multicellular animals possessing effectors only and neither 

 cells sufficiently differentiated to be called receptors nor other nervous mechanisms. 

 The next step in evolution after that of the sponges is the receptor-effector 

 system, as seen in its simplest form in the sea anemones, and, with more complica- 

 tion, in the jelly-fish. The outer surface of a sea anemone is found to be diversely 

 sensitive for different kinds of stimuli and, moreover, the response to stimulation 

 of a tentacle may be a movement in a distant part of the organism, without any 

 movement of intermediate regions, so that something in the nature of nervous 

 transmission is present. On histological examination, the skin of these animals 

 is found to consist of three layers. An outer one contains epithelium cells 

 modified to serve as sense receptors, having bristles on the outer ends. Their 

 inner ends are prolonged into finely branched processes, clearly of nervous nature, 

 which intermingle with those of other cells to form the second nervous layer. 

 This layer also contains cells with branched processes, which intermingle with the 

 rest, in fact, ganglion cells. It appears that this layer constitutes a true nervous 

 network, continuous over the whole body, and that no centralisation of the 

 adjuster mechanism has yet taken place. The third layer consists of muscle cells 

 in contact with the nerve network. Experiments of various kinds show that the 

 reactions of one part of the body do not serve as experience for another, that is, 

 it shows no evidence of what we should call a central nervous system (von 

 TJexkiill, 1909, p. 73). Its neuro-muscular system consists of receptors and 

 effectors, united by a nerve network, which is composed of the processes of 

 receptor cells and of ganglion cells contained in the network. 



The jelly-fish, owing to their locomotion, lead a life subject to greater variety 

 of experiences, and we find specialised forms of receptor organs, which are much 

 more sensitive than those of the sea anemone. The muscular band is under the 

 control of a nerve network, which receives numerous fibres from the receptor 

 organs. This network conveys the excitatory process from one part to another, 

 since contraction can pass from one group of muscle fibres to another over a gap 

 containing network but no muscle fibres. It also conveys excitation in all 

 directions ; if all the sense organs but one are removed, the rhythmic impulses to 

 swimming movements are started by this one and radiate from it in all directions. 



Proceeding upwards, we find in the earthworm a centralised nervous system, a 

 diagram of which, taken from the description and figures of Retzius (1892), is 

 given in Fig. 142. In this animal we have a cerebral ganglion, or brain, in the 



3 



