AVICULARIA AND VIBRACULA 91 I 



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 aviciilaria 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 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 Pedicellarise of 

 Echini, to remove extraneous particles that may be in contact with 

 the surface of the polyzoary. The latter would seem to be the func- 

 tion of the vibracula, which are long bristle-shaped organs (fig. 688, 

 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, 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 tentacles are protruded. 1 



Tunicata. The zoological position of the Timieata, which has 

 long been a subject of great discussion, appears to be now approxi- 

 mately settled ; the study of their development has shown that 

 they are provided with a notochord, and that their nervous system 

 follows the course which is characteristic of what are often called 

 Vertebrata, but should better be called Chordata. As the noto- 

 chord is always restricted to the hinder part of the body, the 

 Tunicata may be called Urochordata. In all (except, perhaps, 

 Appendicularici) there are distinct signs of degeneration. They have 

 been named Tunicata from the inclosure of their bodies in a ' tunic,' 

 which is sometimes leathery or even cartilaginous in its texture, and 

 which sometimes includes calcareous spicules, whose forms are often 

 very beautiful. They are often found to resemble the Polyzoa in 

 their tendency to produce composite structures by gemmation ; but in 

 their habits they are for the most part very inactive, exhibiting 

 scarcely anything comparable to those rapid movements of expansion 

 and retraction which it is so interesting to watch among the Polyzoa ; 

 whilst, with the exception of the Salpidcv and other floating species 

 which are chiefly found in seas warmer than those that surround our 

 coast, and the curious Appendicularla to be presently noticed, they 

 are rooted to one spot during all but the earliest period of their lives. 

 The larger forms of the Ascidian group, which constitutes the bulk 

 of the class, are always solitary ; not propagating by gemmation, 

 except in the case of the Clavelinidae. Although of special importance 



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

 and Vibracular Organs of Polyzoa ' in Trans. Microsc. Soc. ser. ii. vol. ii. 1854, 

 p. 26 ; and Mr. A. W. Waters, ' On the use of the Avicularian Mandible in the Deter- 

 mination of the Cheilostomatous Bryozoa,' Journ. EOIJ. Microsc. Soc. (2), v. p. 774. 



