44 COMPARATIVE ANATOMY. 



external influences. This explains the foldings-in of the portion of 

 the integument which is about to be differentiated into a sensory 

 organ. It is for this reason that the higher sensory organs gradually 

 sink beneath the level of the ectoderm as they are developing, and 

 attain a favourable position for further development. 



§ 38. 



Vesicles filled with a fluid, on the walls of which a nerve ends, 

 are regarded as auditory organs (o to cysts). In its simplest form 

 the vesicle is directly connected to the central nervous system, or the 

 nerve passes from it to the vesicle. These vesicles almost always 

 contain firm concretions or crystalline structures ; and very often 

 crystals of calcic carbonate. There are often hair-like prolongations 

 of the end-organs in addition to them, which project into the lumen 

 of the vesicle. This form of auditory organ, which obtains in the 

 Invertebrata, is complicated in the Vertebrata by diverticula and 

 outgrowths which form a labyrinth. New arrangements are produced 

 in the form of organs for carrying and increasing the sound, which 

 become attached to the auditory organ, although they primitively 

 presided over other functions. Inasmuch as the labyrinth-vesicles 

 of the Vertebrata are developed from the integument, the terminal 

 organs of the auditory nerve which are differentiated in its walls 

 are genetically connected with the terminal organs of the tactile 

 nerves, which lie in the integument; they may therefore be 

 regarded as a specific development of a lower sensory organ. The 

 genetic relations of the simpler otocysts of most Invertebrata are 

 as yet unknown, but all the more exact results point to the supposi- 

 tion that they arise by a differentiation of the ectoderm. 



The optic organ also has a simple mode of origin. We 

 exclude the pigment spots, which used to be often called eyes, and 

 only recognise an eye where a nerve-ending of definite form can be 

 detected, either under or on the surface of the body, acting as an 

 organ for the perception of light. By the light-absorbing property 

 of the pigment it is possible that indefinite sensations of light and 

 shade may be produced, or other sensations altogether unlike that 

 which we call " sight" may possibly be produced by the heat-rays 

 alone of the light. 



The function of pigment in the way just noticed is doubtful, but 

 when it surrounds a part only of a rod-like nerve-ending, and that 

 in such a way as to leave the outermost end free, and exposed alone 

 to the influence of light, it has, clearly enough, a definite function. 

 Optic organs of various degrees of complexity are formed by the 

 union of a few or of many nerve-endings; the elements which 

 are the medium of light-perception (rods) forming a convex or 

 concave layer. Another complication is due to the addition of 

 organs to refract the light (lenses) ; these, too, may have all kinds 

 of relations, but they are always, either directly or indirectly, 

 derived from the integument. In eyes in which the surface of the 



