CLASSIFICATION OF POLYZOA. 499 



bristles, which is probably 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 u avicu- 

 laria" keep up a continual snapping action, during the life of the 

 polyzoary ; 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 in Fig. 247, 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 bodies that may serve as food 

 within the reach of the ciliary current, or whether it is, like the 

 "pedicellaria" of Echini ( 314), 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, each one springing at its base out of 

 a sort of cup (Fig. 245, A), that contains muscles by which it is 

 kept in almost constant motion, sweeping slowly and carefully 

 over the surface of the polyzoary, and removing what might be 

 injurious to the delicate inhabitants of the cells when their ten- 

 tacula are protruded. Out of 191 species of Cheilostomatous 

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

 eitherwith "avicularia," or with "vibracula," or with both of these 

 organs. 1 II. The second order, Cyclostomata, consists of those 

 Polyzoa which have the mouth at the termination of tubular cal- 

 careous cells, without any movable appendage or lip. This 

 includes a comparatively small number of genera, of which Crisia 

 and Tubulipora contain the largest proportion of the species that 

 occur on our own coasts. III. The distinguishing character of 

 the third order, Ctenosomata, is derived from the presence of a 

 comb-like circular fringe of bristles, connected by a delicate 

 membrane, around the mouth of the cell, when the animal is 

 projected from it; this fringe being drawn in when the animal is 

 retracted. The polyzoaries of this group are very various in 

 character, the cells being sometimes horny and separate (as in 

 Laguncula and Bowerbankia\ sometimes fleshy and coalescent (as 

 in Halodactylus). IV. In the fourth order, Pedicellinece, which 

 includes only a single genus, Pedicellina, the lophophore is pro- 

 duced upwards on the back of the tentacles, uniting them at their 

 base in a sort of muscular calyx, and giving to the animal when 

 expanded somewhat the form of an Inverted bell, like that of 



1 See Mr. G. Busk's " Remarks on the Structure and Function of the Avicularian and 

 Vibracular Organs of Polyzoa," in "Transact, of Microscop. Soc." Ser. II, vol. ii, p. 26. 



