Page - 248 - in The Origin of Species

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