. SENSORY ORGANS OF VERTEBRATA. 



523 



other nerves. They form plexuses, which contain a large number of 

 ganglia, and there are also separate ganglion cells, in large numbers, 

 on the course of the sympathetic nerves. 



These plexuses are distributed on the enteric tube, and on all the 

 organs derived from it, as also on the vascular system, and urino- 

 genitary organs. 



This portion of the nervous system appears to be absent in the 

 Acrania, and among the Cyclostomata it is not present in the Myxi- 

 noidea, where the vagus seems to supply the enteric area of the sym- 

 pathetic. From the Pishes onwards this system is always present, 

 although much modified. The fibres of the sympathetic appear to 

 be elements, which permanently retain a lower grade of development, 

 just as do the fibres of the cerebro-spinal nerves of the Cyclostomata. 



Sensory Organs. 



§ 394. 



All the sensory organs of the Vertebrata are formed from differen- 

 tiations of the integument. The kind of share which is taken by the 

 integument differs with the quality of the organ. As in the Inverte- 

 brata,so here we can separate the sensory organs 

 into those which preside over a specific sen- 

 sation, or higher sensory organs, and those 

 which appear to serve for the various percep- 

 tions, which are indifferent in character and 

 may be regarded as belonging to the sense of 

 touch. 



There are many organs which cannot be 

 reckoned among the known specific sensory 

 organs, but which are remarkable for the high 

 grade of their differentiation, so that their ar- 

 rangements do not allow us to regard them as 

 mere "tactile organs;'^ these seem to justify us 

 in assuming the existence of specific sensory 

 organs, other than those known to us familiarly. 



Organs of this kind are most varied in 

 character among Fishes, and as many of these 

 arrangements are repeated in the Amphibia it 

 is probable that they are correlated with an 

 aquatic life. The most important organs of 

 this kind are the following : 



1. Goblet-shaped organs. Large struc- 

 tures embedded in the epidermal layer, and 

 surrounded by long spindle-shaped cells ; they 

 enclose rod-shaped end-organs of nerves ; they have been observed 

 in the skin of the Teleostei and of the Sturgeon, and appear to 



Fig. 291. Goblet-shaped 

 organs from the mucous 

 membrane of the palate 

 of Tinea. n Nerve- 

 trunk. 6 Goblet (after 

 F. E. Schulze). 



