346 PHYSIOLOGY OF THE INVERTEBRATA. 



tions, and it is only by the senses of taste, smell, hearing, 

 and sight, that this intermediate organ between the nerve 

 and the external world is a necessary condition. 



We now proceed to describe the sense-organs in the 

 principal divisions of the Invertebrata. 



_The Protozoa. 



As these organisms are destitute of any true nervous 

 system, it would be consistent, on d, priori grounds, to assume 

 that they have no special sense-organs. But would it be 

 consistent to assume that these lowly organisms do not digest, 

 respire, and excrete, because there are present no special 

 organs set apart for the functions of digestion, respiration, 

 and excretion ? Certainly not, and there is every reason to 

 believe that one or more of the special senses are represented 

 in the Protozoa. 



Tactile sensibility is generally distributed over the whole 

 surface of the body ; frequently, however, it is concentrated 

 on processes and appendages of it. This is more or less true 

 in the whole animal kingdom. In the Protozoa, the whole 

 surface of the body is exceedingly sensitive ; but it may be 

 stated that the protoplasmic expansions called pseudopodia 

 have been regarded as fulfilling the function of organs of 

 touch as well as of locomotion. In other forms {e.g., Para^ 

 mmcium, see Pig. 3) the vibrating cilia are considered by 

 Dr. Stein to be organs of touch ; and the long rigid bristle 

 in Gryptocliilwn, according to M. Maupas, has a similar 

 function, its principal use being " to advise the animal of the 

 approach of other Infusoria." 



In touch, sensibility is brought into play by simple 

 shock, or contact of bodies : it is spoken of as the least 

 perfect of the senses, and is also the one,' which offers the 

 least variety in the different animal classes, compared among 

 themselves. 



Of all the sense-organs, the eye is the one which is first 



