852 
MR. G. J. ROMANES AND PROFESSOR J. C. EWART 
that the presence of such organs in these other Echinodermata raises no real difficulty 
in the way of accepting the proof which we have rendered of their observed functions 
as they occur in their most efficient forms. 
Concerning the physiology of the pedicellariae little further remains to be said under 
the present section. It may be stated, however, that the mandibles, which are con¬ 
stantly swaying about upon their contractile stalks as if in search for something to 
catch, will snap at an object only if it touches the inner surface of one or more of the 
expanded mandibles. Moreover, in the larger pedicellariae, a certain part of the inner 
surface of the mandibles is much more sensitive to contact than is the rest of that 
surface ; this part is a little pad about one-third of the way down the mandible 
(Plate 80 , fig. 13, s); a delicate touch with a hair upon this part of any of the three 
mandibles is certain to determine an immediate closure of all the three. It is obvious 
that there is an advantage in the sensitive area, or zone, being placed thus low enough 
down in the length of the mandibles to ensure that the whole apparatus will not close 
upon an object till the latter is far enough within the grasp of the mechanism to give 
this mechanism the best possible hold. If, for instance, the tips of the mandibles were 
the most sensitive parts, or even if their whole inner surfaces were uniformly sensitive, 
the apparatus would be constantly closing upon objects when these merely brushed 
past their tips, and therefore closing prematurely for the purpose of grasping. But, 
as it is, the apparatus is admirably adapted to waiting for the best possible chance of 
getting a secure hold, and then snapping upon the object with all the quickness and 
tenacity of a spring-trap. 
Another point worth mentioning is that if, after closure, any one or more of the 
mandibles be gently stroked on its outer surface near the base, all the mandibles are 
by this stimulation usually, though not invariably, induced again to expand. This is 
the only part of the whole organ the stimulation of which thus exerts an inhibitory 
influence on the contractile mechanism. If there is any functional purpose served by 
this relaxing influence of stimulating this particular part of the apparatus, we think it 
can only be as follows. When a portion of sea-weed brushes this particular part, it 
must be well below the tips of the mandibles, and therefore in a position where it, or 
some over-lying portion, may soon pass between the mandibles, if the latter are open ; 
hence when touched in this place the mandibles, if closed, open to receive the sea-w T eed, 
should any part of it come within their cavity. 
3. Physiology of the Spines. —We must next consider stimulation with reference to 
the spines. It has already been said that these organs co-operote with the pedicellarise 
in grasping any instrument of stimulation, and this proves that for a certain area round 
any seat of stimulation the spines admit of co-ordinated action. Further experiments 
prove that there is no limit to the area within which co-ordinated action of the spines 
may take place ; but that all the spines of the organism may work together to the 
attainment of some common end. Thus it has already been stated in a previous part 
of this paper that a Spatangus, when placed upon its back, is able to right itself by 
