248 ORIGIN OF SPECIES 



give a fuller abstract of Mr. Agassiz's interesting observa- 

 tions on the development of the pedicellarise. All possible 

 gradations, as he adds, may likewise be found between the 

 pedicellariae of the star-fishes and the hooks of the Ophiuri- 

 ans, another group of the Echinodermata ; and again between 

 the pedicellariae of sea-urchins and the anchors of the Holo- 

 thurise, also belonging to the same great class. 



Certain compound animals, or zoophytes as they have been 

 termed, namely the Polyzoa, are provided with curious or- 

 gans called avicularia. These differ much in structure in the 

 different species. In their most perfect condition, they curi- 

 ously resemble the head and beak of a vulture in miniature, 

 seated on a neck and capable of movement, as is likewise the 

 lower jaw or mandible. In one species observed by me all the 

 avicularia on the same branch often moved simultaneously 

 backwards and forwards, with the lower jaw widely open, 

 through an angle of about 90°, in the course of five seconds; 

 and their movement caused the whole polyzoary to 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 pedicellariae of the Echinodermata, which 

 he considers as "essentially 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 avicu- 

 laria. The latter resemble somewhat more closely the chelae 

 or pincers of Crustaceans; and Mr. Mivart might have ad- 

 duced 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, however, does not know of any 

 gradations now existing between a zooid and an avicularium. 

 It is therefore impossible to conjecture by what serviceable 



