THE SENSE-ORGANS 473 



ment of this system, the part to which it is distributed must 

 have been so also. A suggestive comparison has also been 

 made between the semicircular canals of the inner ear, each 

 with its own ampulla, and the canals of Lorenzini, the resem- 

 blance between which is apparent. The actual value of this 

 comparison is somewhat questionable, and the theory itself, al- 

 though far better supported by the facts than are either of 

 the others, is seriously opposed by the actual developmental 

 history of the labyrinth ; which arises as a vesicle invaginated 

 from the exterior long previous to and not associated with the 

 lateral line organs. The otic vesicle seems rather to form one 

 of a series of very early organs, to which belong also the lens 

 of the eye and possibly the nasal sacs, as well as a few transi- 

 tory structures associated with other cranial nerves and usually 

 interpreted as lost sense-organs of unknown function. In our 

 present state of knowledge it. seems a surer course to believe 

 that the entire lateral line system of the Ichthyopsida, the 

 function of which is in some way associated with an aquatic 

 habitat, disappears completely where the assumption of a ter- 

 restrial life renders it no longer necessary. 



In connection with the description of the general tactile 

 sense, that of the so-called " free nerve endings," certain more 

 specialized forms of nerve termini were referred to, which in 

 our present lack of precise knowledge are classed under the 

 general head of organs of touch. The most elementary of these 

 are the tactile cells (Fig. 128, b), which are scarcely more than 

 isolated units of the general type, somewhat more specialized 

 and thus rendered conspicuous. They are first seen in tailless 

 amphibians, where they are associated in groups, forming small 

 areas known as tactile spots. In other cases the ending has a 

 tendency to form a bulb or sphere, composed of many cells,, 

 and often of appreciable size; these are termed collectively 

 tactile corpuscles, each different type being designated by the 

 name of the investigator who first made an accurate description 

 of it. These tactile corpuscles show various types of structure 

 and make use of very different mechanical principles. Thus, 

 in Meissner's corpuscles (Fig. 128, c) the terminal cells form 



