Pedicellarice in the Echinidse. 107 



side of the median line, and open at the apex of the valve 

 on the sides of the fang as above mentioned, whence the 

 discharge of the mucous or glairy matter secreted by the 

 glands of the sac takes place. This sac (or double sac more 

 accurately) is placed external to, or upon the outward side of, 

 the central calcareous shaft of the jaw -piece. The three 

 valves are held together by strong bands of muscles, which 

 are attached within the hollow cavity of the basal portion, 

 the muscular impressions being clearly perceptible on either 

 side of the median ridge after the calcareous skeleton has 

 been cleaned in a strong potash solution. Nerve-centres 

 with fibres running into each valve, communicating with 

 these muscles, may be traced in the middle of the basal por- 

 tion of the head immediately above the upper extremity of the 

 central shaft of the stem. 



When the jaws or valves of the pedicellaria are opened 

 wide, the inner margin of the base of the internal skeleton, 

 the lower part of the free edge of the partitional keel of 

 its basal cavity, and the extreme tip of the fang, all fall in 

 one and the same plane, which is at right angles to the stem 

 of the pedicellaria (PI. XII. fig. 4). Owing to the stretching 

 of the investing membrane a flat surface of triangular outline 

 is thus presented, which resembles fancifully the contour of 

 an old-fashioned three-cornered hat, the edges being some- 

 what loose and overturned (PI. XII. fig. 5) . In the centre 

 of this inner surface of the expanded valves are three, small, 

 oval-shaped cushions, approximated at their bases, and one 

 lying upon the median keel of each of the valve-frames 

 (PI. XII. fig. 5, and PI. XIII. figs. 9, 10). These organs, 

 which are finely papillate and richly supplied with nerve- 

 fibres (as will be found indicated in the section in PI. XIII. 

 fig. 12), are presumably of sensorial {i.e. tactile) function, and 

 act, in all probability, as the communicators of the advent of 

 any foreign or irritating element. When the Echinus is alive 

 and at rest in a congenial environment the jaws of the pedi- 

 cellaria are extended, the whole appendage swaying gently 

 to and fro as if in search of, and in readiness for, a coming- 

 enemy. 



Functions of the Pedicellarice globiferce. — Respecting the 

 functions of pedicellaria3, around which subject there still 

 remains so much doubt, I shall, in the present communica- 

 tion, chiefly confine my remarks to the ped. globiferce — the 

 form that has been under examination. 



It would appear that one of the principal physiological 

 actions, and that which distinguishes the present from each 

 of the other forms of pedicellariee, consists in the discharge of 



