STAR-FISH AND SEA-UP.CHINS. 319 
spines, and pedicellariz together, so that they all 
approximate a point of irritation situated anywhere 
in that plexus; (2) of an internal nervous plexus 
which is everywhere in communication through the 
thickness of the shell with the external, and the 
function of which is that of bringing the feet, spines, 
and probably also the pedicellariz into relation 
with the great co-ordinating nerve-centre situated 
round the mouth; (3) of central nervous matter 
which is mainly gathered round the mouth, and 
there presides exclusively over the co-ordinated 
action of the spines, and in large part also over 
the co-ordinated action of the feet, but which is 
further in part distributed along the courses of the 
main nerve-trunks, and so secures co-ordination of 
feet even in separated segments of the animal. 
Special Senses. 
Before concluding, I must say a few words on 
the experiments whereby we sought to test for the 
presence in Echinoderms of the special senses of 
sight and smell. 
We have found unequivocal evidence of the Star- 
fish (with the exception of the Brittle-stars) and 
the Echini manifesting a strong disposition to crawl 
towards, and remain in, the light. Thus, if a large 
tank be completely darkened, except at one end 
where a narrow slit of light is admitted, and if a 
number of Star-fish and Kchini be scattered over 
the floor of the tank, in a few hours the whole 
number, with the exception of perhaps a few per 
