108 Mr. W. Percy Sladen on the 



mucus. This takes place, in the species we have been dis- 

 cussing (viz. Sphcerechinus granularis) , both from the head 

 of the pedicellaria and from the sacculi of the stem. 



A careful study of these organs in the living state, and a 

 comparison of the habits of other Eehinoderms, have led me 

 to formulate the following explanation of the functions sub- 

 served by the pedicellarice globiferce ) and although I am not 

 at present able to do more than offer it as a suggestion, the 

 facts that have laid the foundation for this conclusion are in- 

 controvertible, whilst the argument deduced therefrom would 

 appeal to the judgment by virtue of its reasonableness alone, 

 even if it were unsupported by the collateral evidence of 

 which we are in possession. 



It has previously been remarked that a Sphcerechinus 

 granularis j when placed in fresh water, throws out a glairy 

 mucous matter that envelops the test ; and the author has 

 seen the same discharge take place when the urchin was 

 placed in a vessel of sea- water different from that which it 

 had previously occupied. This discharge comes from the 

 jpedicellarke globiferce\ and the opinion has already been 

 stated that these appendages are the source to which it is 

 entirely due. 



The reason of the discharge is evidently an attempt on the 

 part of the animal to free itself from unpleasant surroundings. 

 The prime cause, I am inclined to think, lies in the explana- 

 tion that the function of these pedicellaria? is that of removing 

 grit, sand, dirt, or other offensive matter from the surface of 

 the animal. 



From the manner of the arrangement of parts upon the 

 surface of a sea-urchin, it can readily be imagined that a 

 natural tendency must exist towards the accumulation of all 

 such substances amongst the many crevices and interstices 

 on the body-surface, between crowded spines, ambulacral 

 suckers, and pedicellaria?. Yet it will be found that the 

 Echinus is generally wonderfully clean. In my opinion the 

 ciliary epithelium is altogether insufficient to effect this ; 

 and the duty devolves upon the pedicellaria} globiferce, 

 the following being the manner in which the work is per- 

 formed. 



When the tactile cushion of the pedicellaria comes into con- 

 tact with a tangible object of foreign matter, the valves close 

 and a discharge of mucus takes place, wherewith the ob- 

 noxious object is covered. When the hold of the jaws is 

 again relaxed the irritating substance remains entangled in a 

 cloud of the glairy exudation, ready to be easily disengaged from 

 the surface of the animal by a few movements of the neigh- 



