226 MISCELLANEOUS OBJECTIONS TO TEE 



a tridactyle forceps — that is, of one formed of three ser« 

 rated arms, neatly fitting together and placed on the 

 summit of a flexible stem, moved by muscles. These for- 

 ceps can seize firmly hold of any object; and Alexander 

 Agassiz has seen an Echinus or sea-urchin rapidly passing 

 particles of excrement from forceps to forceps down certain 

 lines of its body, in order that its shell should not be 

 fouled. But there is no doubt that besides removing dirt 

 of all kinds, they subserve other functions; and one of 

 these apparently is defence. 



With respect to these organs, Mr. Mivart, as on so many 

 previous occasions, asks: '^ What would be the utility of 

 the first rudimentary hegimiings of such structures, and 

 how could such insipient buddings have ever preserved the 

 life of a single Echinus?" He adds, ^'^not even the sudden 

 development of the snapping action could have been bene- 

 ficial without the freely movable stalk, nor could the 

 latter have been efficient without the snapping jaws, yet 

 no minute, merely indefinite variations could simultane- 

 ously evolve these complex co-ordinations of structure; to 

 deny this seems to do no less than to affirm a startling 

 paradox." Paradoxical as this may appear to Mr. Mivart, 

 tridactyle forcepses, immovably fixed at the base, but capa- 

 ble of a snapping action, certainly exist on some star-fishes; 

 and this is intelligible if they serve, at least in part, as a 

 means of defence. Mr. Agassiz, to whose great kindness 

 I am indebted for much information on the subject, in- 

 forms me that there are other star-fishes, in which one of 

 the three arms of the forceps is reduced to a support for 

 the other two; and again, other genera in which the third 

 arm is completely lost. In Echinoneus, the shell is de- 

 scribed by M. Perrier as bearing two kinds of pedicellariae, 

 one resembling those of Echinus, and the other those of 

 Spatangus; and such cases are always interesting as afford- 

 ing the means of ajoparently sudden transitions, through 

 the abortion of one of the two states of an organ. 



With respect to the steps by which these curious organs 

 have been evolved, Mr. Agassiz infers from his own re- 

 searches and those of Mr. Miiller, that both in star-fishes 

 and sea-urchins the pedicellariBe must undoubtedly be 

 looked at as modified spines. This may be inferred from 

 their manner of development in the individual, as well as 



