Chap. XII.] 



SENSORY ORGANS. 



remain throughout the life of the animal in close local 

 contact with the outer world. In tracing the history 

 of the organs of sense we shall find that, whatever 

 their final position, they too are essentially of epiblastic 

 origin.* 



Among the Hydroid polyps, where no nervous 

 system has as yet been made out, we observe that the 

 tentacles which surround the mouth are provided with 

 fine hair-like projections, which look not unlike a 

 trigger ; these processes are seen to be in connection 

 with cells which differ in character from their neigh- 

 bours by the possession of a coiled up thread; when 



* Since the above was put into type, Prof. Charles Stewart 

 has favoured me with an account of his observations on sense cells 

 in sponges, and with the accompanying illustrative figure (Fig. 

 184 A). It is found that 

 '' the external orifices 

 of the interradial canals 

 of Grantia comjiressfi 

 are fringed with deli- 

 cate hair-like processes 

 of the soft substance of 

 the sponge. At first 

 sight these remind one 

 of the palpocils of Hy- 

 dra, which they closely 

 resemble in general 

 form and size " ; from 

 these, however, they 

 differ in important par- 

 ticulars. The processes 

 or hairs vary in length 

 from 3^-jj^th to about 

 j-cVjjth of an inch ; 

 their base is from -g^^th to Ttr&rnj'kl 1 f an inch, and they taper 

 to a fine point. All such as can be well seen are found to have 

 a special relation to a subjacent branched cell ; this latter sends 

 outwards a delicate filament which traverses the axis of the pro- 

 cess. " Such an apparatus appears both by position and structure 

 to be specially impressed by varying conditions in the inrushing 

 water, particles in solution or suspension in this water inducing 

 molecular changes in the cell at the base of the process, and per- 

 haps leading to the contraction of neighbouring cells. In other 

 words, these processes seem to act as part of an automatic 

 mechanism for regulating the water-currents of the organism." 



Fig. 184 A. 



