POLYZOA AND TtJNICATA. 



163 



FIG. 381. 



margins of the cells, that is, are attached at once to them without 

 the intervention of a stalk, as at A, being either ' projecting ' or 

 'immersed;' but in the genera 

 Bugula and Bicellaria, where 

 they are present at all, they are 

 ' pedunculate/ or mounted on 

 footstalks (B). Under one form 

 or the other, they are wanting in 

 but few of the genera belonging 

 to this order; and their presence 

 or absence furnishes valuable char- 

 acters for the discrimination of 

 species. Each avicularium has 

 two ( mandibles/ of which one is 

 fixed, like the upper jaw of a 

 bird, the other movable, like its 

 lower jaw; the latter is opened and 

 closed by two sets of muscles which 

 are seen in the interior of the 

 'head;' and between them is a 

 peculiar body, furnished with a 

 pencil of bristles, which is prob- 

 ably a tactile organ, being brought 

 forwards when the mouth is open, 

 so that the bristles project beyond 

 it, and being drawn back when the 



mandible Closes. The avicularia A> Portion of cellularia ciUata, enlarged; B, one 

 .keep Up a Continual Snapping aC-of the 'bird's-head' processes of Bugula avicularia, 



tion during the life of the P o1y Z o-S'S 1 h 1 1 h e 1 r ymagnmed ' and8eenlntheaotof 8ra8p ' 

 ary ; and they may often be observed 



to lay hold of minute Worms or other bodies, sometimes even closing upon 

 the beaks of adjacent organs of the same kind, as shown at B. In the 

 pedunculate forms, besides the snapping action, there is a continual 

 rhythmical nodding of the head upon the stalk; and few spectacles are 

 more curious than a portion of the polyzoary of Bugula avicularia (a very 

 common British species) in a state of active vitality, when viewed under 

 a power sufficiently low to allow a number of these bodies to be in sight 

 at once. It is still very doubtful what is their precise function in the 

 economy of the animal; whether it is to retain within the reach of the 

 ciliary current, bodies that may serve as food; or whether it is, like the 

 PedicellariaB of Echini ( 534), to remove extraneous particles that may 

 be in contact with the surface of the polyzoary. The latter would seem 

 to be the function of the vibracula, which are long bristle-shaped organs 

 (Fig. 380, A), each one springing at its base out of a sort of cup that 

 contains muscles by which it is kept in almost constant motion, sweep- 

 ing slowly and carefully over the surface of the polyzoary, and removing 

 what might be injurious to the delicate inhabitants of the cells when 

 their tentacles are protruded. Out of 191 species of Cheilostomatous 

 Polyzoa described by Mr. Busk, no fewer than 126 are furnished either 

 with Avicularia, or with Vibracula, or with both these organs. 1 



555. TU^ICATA. The Tunicated Mollusca are so named from the in- 



1 See Mr. G-. Busk's Remarks on the Structure and Function of the Avicula- 

 rian and Vibracular Organs of Polyzoa,' in " Transact, of Microsc. Soc.," Ser. 2, 

 Vol. ii. (1854), p. 26. 



