104 



motile portion, the mandible, as I have termed it in the one case, 

 and the seta in the other. 



The avicularium, besides the moveable mandible, which varies 

 extremely in form and length, always has a more or less well- 

 marked, corresponding fixed beak, the opponent as it w r ere of the 

 mandible and serving to constitute the organ an instrument of 

 prehension. 



This beak is necessarily absent in the vibraculum, which appears 

 to be merely a defensive organ, or perhaps in some cases (as in the 

 Selenariadce) having a locomotive function, and its absence in 

 cases where the moveable part of the organ is detached would 

 serve to distinguish the one kind of organ from the other. It is 

 to be regretted, however, that in the instance of fossil species, 

 where such a character would often be of extreme value, this 

 beak, which is, I believe, always constituted of a horny or chiti- 

 nous substance, is removed together with all other vestiges of the 

 animal tissues. The presence of a beak also serves to determine 

 in some cases the avicularian nature of the organ, when, as in 

 several species of Lepralia, the setose form of the mandible 

 might indicate that it would be more correctly referred to the 

 vibracular type. 



1. As regards the avicularia. 



These organs, which appear to have been first noticed by Ellis, 

 have been investigated and described by many subsequent 

 authors, among whom are particularly to be noticed Mr. Darwin, 

 Dr. Van Beneden, the late Professor John Reid, and especially 

 Nordmann and Krohn. 



In some observations upon the structure of the polyzoary and 

 other points in the economy of Notamia bursaria, published in 

 the Transactions of the Microscopical Society for 1847, I de- 

 scribed more particularly the structure of the curious and unique 

 form presented by the avicularium in that Polyzoan, pointing 

 out I believe for the first time, that the muscles were divisible 

 into two distinct sets, one for the closure and the other for the 

 opening of the mandible ; I also indicated that the mandible and 

 beak were constituted of a different substance to the rest of the 

 organ, as above adverted to ; and that, besides the two sets of 

 muscles, the cup contained a "peculiar body of unknown na- 

 ture." 



I have in addition only to remark that since then it has oc- 

 curred to me to notice a circumstance hitherto overlooked, and 

 which may eventually serve to throw some* light upon the " pecu- 

 liar body " contained in the cell to which I adverted in my ob- 

 servations on Notamia. It was in that species, also, that I first 

 noticed the fact that when the mandible is thrown back, or in 

 other words, when the avicularium is open, a slight prominence 

 comes into view, covered with delicate seta, which do not seem 



