THE ORGANS. 43 



Their presence therefore presupposes that form of differentiation 

 which we treated of above, when speaking of the nervous system. 

 Since ontogenetic facts point to the primitive segregation of the 

 nervous system from the ectoderm, as being most probably a funda- 

 mental process, this same outermost layer of the body becomes also 

 of the greatest importance in studying the origin of the sensory 

 organs. Almost all the sensory organs are derived, directly or 

 indirectly, from it; whence arises the permanent or temporary 

 connection of these organs with the integument. 



It is very unsafe to assert what are the functions of many of the 

 sensory organs of the lower animals. This applies to all those organs 

 which are not comprised amongst those which fall within the domain 

 of our own judgment, on account of our possessing them or their 

 homologues, in which case only is it possible that the connection 

 between their structure and specific function can be estimated. 

 Such outstanding organs have been classed together as organs of a 

 sixth sense. 



§ 37. 



The sensory organs are divided into lower and higher. The 

 former are commonly distributed over the integument, and are simple 

 iu structure. Compared with the higher they represent a more 

 indifferent condition. Modified cells of the integument, which 

 generally belong to the epidermis, connected on the one hand with 

 a nerve fibre, and on the other provided with a process of varying 

 shape, which is directed towards the surface of the body, are the 

 most common examples of the lower sort. They are regarded as 

 the organs of general tactile perception ; but the physiological 

 function of these organs, especially in aquatic animals, has not been 

 definitely determined, and it is reasonable to suppose that many of 

 them are the media of specific sensations, in which case they would 

 resemble the higher organs of sense. 



The significance of these arrangements is somewhat more certain 

 when they are connected with special organs, such as movable 

 processes of the integument and the like ; they then appear to be 

 tactile organs. It is still a question whether structures of this 

 kind, especially in the lower divisions of the Animal Kingdom, are 

 of use for perceptions other than tactile. 



The higher sensory organs present themselves to us as special 

 elaborations, with one special function and capable of response only 

 to stimuli of one special kind ; they are to be regarded as developed 

 from the lower kind of sensory organs, and oftentimes still possess 

 the essential structure of that lower kind. Organs of taste and 

 smell can only be certainly distinguished in the higher divisions of 

 the Animal Kingdom, and the function of the latter is certain only in 

 those Vertebrata which live in the air ; in the lower divisions it is 

 still doubtful. Even in the case of the organs of taste the greatest 

 caution is necessary as to their real import. The value of a sensory 

 organ to the organism determines its being protected against 



