332 



F. VOLUTION OF THE NERVOUS SYSTEM. 



cells sensitive to light, and of ganglion-cells connected witli them ; while at 

 a later period optical structures, constituting a lens capable of throwing 

 an image of external objects upon it, would be developed, and ko convert 

 the whole structvire into a true organ of vision. It has thus come about 

 that, in the development of the individual, the retina is often first formed in 

 connection with the central nervous system, while the lenses of the eye are 

 independently evolved from the epidermis at a later period. 



A series of forms of the Ccslenterata and Platyelminthes affords us 

 examples of various stages in the differentiation of a central nervovis 

 system \ 



In sea-anemones (Hertwigs, No. 321) there are, for instance, no organs 

 of special sense, and no definite central nervous system. There are, how- 

 ever, scattered throughout the skin, and also throughout the lining of the 

 digestive tract, a number of specially modified epithelial cells, which are 

 no doubt delicate organs of sense. Tliey are provided at their' free 

 extremity with a long hair, and ai-e prolonged on their inner side into 

 fine processes which penetrate into the deeper part of the epithelial layer 

 of the skin or digestive wall. They eventually join a fine network of 

 protoplasmic fibres which forms a special layer im- 

 mediately within the epithelium. The fibres of this 

 network are no doubt essentially nei vous. In addition 

 to fibres there are, moreover, present in the network 

 cells of the same character as the multipolar ganglion- 

 cells in the nervous system of Vertebrates, and some 

 of these cells are characterised by sending a process 

 into the superjacent epithelium. Such cells are ob- 

 viously intermediate between neuro-epithelial cells and 

 ganglion-cells ; and it is probable that the nerve-cells 

 are, in fact, seni<e-cells which have travelled inwards 

 and lost their epithelial character. 



In the Craspedote Medusae (Hertwigs, No. 320) the . 

 differentiation of the nervous system is carried some- 

 what further. Thei'e is here a definite double ring, 

 placed at the insertion of the velum, and usually con- 

 nected with sense-organs. The two parts of the ring 

 bslong respectively to the epithelial layers on the upper 

 and lower surfaces of the velum, and are not separated 

 from these layers ; they are formed of fine nerve- 

 fibres and ganglion- cells. The epithelium above the 

 nerve rings contains sense-cells (fig. 237) with a stiff 

 hair at their free extremity, and a nervous prolongation at the opposite end, 

 which joins the nerve-fibres of the ring. Between such cells and true 

 ganglion-cells an intermediate type of cell has been found (fig. 237 B) 

 which sends a process upwards amongst the epithelial cells, but does not 

 reach the surface. Such cells, as the Hertwigs have pointed out, are clearly 

 sense-cells partially ti-ansformed into ganglion-cells. 



A still higher type of nervous system has been met with amongst some 

 primitive Nemertines (Hnbrecht, No. 323), consisting of a pair of large 



^ Our knowledge on this subject is especially due to the brothers Hertwig (Nos. 320 

 and 321), Elmer (No. 318), Glaus (No. 317), Schiifer (No. 326), and Hubrecht (No. 323). 



Fig. 236. Neueo- 

 epithelial sense- 

 cells of aurelia 

 AURiTA. (From Lan- 

 kester ; after Schii- 

 fer.) 



