*Chap. VII. Theory of Natural Selection. 193 



tremble. When the jaws are touched with a needle they seize it 

 so firmly that the branch can thus be shaken. 



Mr. Mivart adduces this case, chiefly on account of the supposed 

 difficulty of organs, namely the avicularia of the Polyzoa and the 

 pedicellarise of the Echinodermata, which he considers as " essen- 

 tially similar," having been developed through natural selection in 

 widely distinct divisions of the animal kingdom. But, as far as 

 structure is concerned, I can see no similarity between tridactyle 

 pedicellariae and avicularia. The latter resemble somewhat more 

 closely the chelse or pincers of Crustaceans ; and Mr. Mivart might 

 have adduced with equal appropriateness this resemblance as a 

 special difficulty ; or even their resemblance to the head and beak 

 of a bird. The avicularia are believed by Mr. Busk, Dr. Smitt, and 

 Dr. Nitsche — naturalists who have carefully studied this group — to 

 be homologous with the zooids and their cells which compose the 

 zoophyte ; the moveable lip or lid of the cell corresponding with the 

 lower and moveable mandible of the avicularium. Mr. Busk, how- 

 ever, does not know of any gradations now existing between a zooid 

 and an avicularium. It is therefore impossible to conjecture by 

 what serviceable gradations the one could have been converted into 

 the other: but it by no means follows from this that such grada- 

 tions have not existed. 



As the chela3 of Crustaceans resemble in some degree the avicu- 

 laria of Polyzoa, both serving as pincers, it may be worth while to 

 -show that with the former a long series of serviceable gradations 

 still exists. In the first and simplest stage, the terminal segment 

 of a limb shuts down either on the square summit of the broad 

 penultimate segment, or against one whole side ; and is thus enabled 

 to catch hold of an object ; but the limb still serves as an organ 

 of locomotion. We next find one corner of the broad penultimate 

 segment slightly prominent, sometimes furnished with irregular 

 teeth ; and against these the terminal segment shuts down. By an 

 increase in the size of this projection, with its shape, as well as that 

 of the terminal segment, slightly modified and improved, the pincers 

 are rendered more and more perfect, until we have at last an instru- 

 ment as efficient as the chelse of a lobster ; and all these gradations 

 can be actually traced. 



Besides the avicularia, the Polyzoa possess curious organs called 

 vibracula. These generally consist of long bristles, capable of 

 movement and easily excited. In one species examined by me 

 the vibracula were slightly curved and serrated along the outer 

 margin ; and all of them on the same polyzoary often moved simul- 

 taneously ; so that, acting like long oars, they swept a branch rapidly 



o 



