582 polyzoa : — avicularia and vibracula. 



the form of an inverted bell, like that of Vorlicella (Fig. 239). — 

 The cells of the Mjtyocrepia or Fresh-water Polyzoa are for the 

 most part lodged in a sort of gelatinous substratum, which spreads 

 over the leaves of Aquatic Plants, sometimes forming masses of 

 considerable size; but in the very curious and beautiful Cristatella, 

 the Polyzoary is unattached, so as to be capable of moving freely 

 through the waters. — As the Polyzoa altogether resemble the true 

 Zoophytes in their habits, and are found in the same localities, it is 

 not requisite to add anything to what has already been said (§§ 419, 

 420), respecting the collection, examination, and mounting, of this 

 very interesting class of objects.* 



451. A large proportion of the Polyzoa of the first Order are 

 furnished with very peculiar motile appendages, which are of two 

 kinds, Avicularia and Vibracula. The Avicularia or ' bird's-head 

 processes,' are so named from the striking resemblance they present 

 to the head and jaws of a Bird (Fig. 301, b). They are generally 

 ' sessile ' upon the angles or margins of the cells, that is, are 

 attached at once to them, without the intervention of a stalk, as in 

 Fig. 301, 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 

 characters 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 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 

 Avicularia 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. 301, 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 



* For a more detailed account of the Structure and Classification of 

 this group, see Prof. Van Beneden's ' Recherches sur les Bryozoaires de 

 la Cote d'Ostende,' in "Me"m. de l'Acad. Roy. de Bruxelles," torn, xvii.; 

 Mr. G. Busk's "Catalogue of the Marine Polyzoa in the Collection of the 

 British Museum ; " Mr. Huxley's ' Note on the Reproductive Organs of 

 the Cheilostome Polyzoa,' in "Quart. Journ. of Microsc. Sci.," Vol. iv. 

 p. 191 ; Dr. G. Johnson's "History of British Zoophytes;" and Prof. 

 Allman's beautiful " Monograph of the British Fresh-water Polyzoa," 

 published by the Ray Society, 1857. 



