43 



internal organs, these movements consist indeed in an essentially passive rising and sinking 

 of the dorsal skin, accordingly as the fluid contained in the perivisceral cavity is more con- 

 centrated in the cavity of the disc or in that of the arms, whereby the roof of the disc 

 becomes alternately rather convex or vaulted, and then quite flat again; moreover there is 

 a contraction and extension of the bucal membrane, which takes place within very wide limits, 

 and whereby the form and size of the bucal aperture can be materially altered. 



B Sensation. 



I have already described as true instruments of sense the peculiar terminal organs 

 of the arms (Tab. II, fig. 5, 6 c, fig. 7 a) the structure and peculiar protection of which 

 distinguish them from all the other appendages of the body. I have previously developed 

 my reasons for not supposing them to be simply and solely organs of feeling or tentacles. 

 Although it is of course very difficult to decide as to the nature of the sensation received 

 through these organs, there is however reason to presume that it must be of a more 

 specific sort than the more indifferent sensation of feeling. If I have been disposed 

 to consider them as something like organs of smell, it was only because in their struc- 

 ture they seemed to me to correspond to what in other invertebrate animals are usually 

 called olfactory papilla?, but I do not therefore mean to say that the sensation conveyed 

 through these organs must exactly correspond to what we understand by smell. There is 

 naturally no objection to our assuming that in several of the lower animals there may be 

 senses of an intermediate sort impossible to class precisely as any of the 5 human senses. 

 "With regard to the position of these organs of sense in the Brisinga, is is evident that in 

 occupying the extreme point of the arms they have got the most convenient place which 

 they could have had. The animal is hereby enabled to explore the immediate environs in 

 all directions at the same time, and indeed over a very considerable area (more than 4 

 square feet); and whether the presence of suitable food or of inconvenience or danger be 

 thus discerned, the animal will easily receive information through one or other of these 

 organs and regulate its movements accordingly. 



As real organs of feeling or tentacles (exclusively of their functions as organs of 

 locomotion previously noticed) we must undoubtedly consider the numerous water-feet at- 

 tached along the ambulacral furrows, which by their great contractility and mobility in all 

 directions, and by their fine cuticular covering, seem to be admirably adapted for their 

 functions in such capacity. All naturalists are also agreed in attributing both the functions 

 mentioned to these organs in other star-fishes. 



It is also extremely probable that the general cuticular covering, and especially the 

 exterior layer of skin in the Brisinga, may be susceptible of a rather fine sensation of a 

 more general kind. 



