9io 



POLYZOA AND TUNICATA 



that of Vorticella (fig. 593). As the Polyzoa altogether resemble 

 hydroid zoophytes in their habits, and are found in the same localities, 

 it is not requisite to add anything to what has already been said 

 respecting the collection, examination, and mounting of this very 

 interesting class of objects. 1 



A large proportion of the Cheilostomata are furnished with very 

 peculiar motile appendages, which are of two kinds, avicularia and 

 vibracula. The avicularia or ' bird's head processes,' so named from 

 the striking resemblance they present to the head and jaws of a bird 



(fig. 689, B), are generally, 

 when highly differentiated, 

 ' sessile ' upon the angles 

 or margins of the cells, 

 that is, are attached at 

 once to them without the 

 intervention of a stalk, as 

 at A, being either * pro- 

 jecting' or ' immersed ; ' but 

 in the genera Bugula and 

 Bicellaria, where they are 

 present at all, they are 

 ' pedunculate,' or mounted 

 on foot-stalks (B). Under 

 one form or the other, they 

 are wanting in but few of 

 the genera belonging to 

 this order ; and their pre- 

 sence or absence furnishes 

 valuable characters for the 

 discrimination of species. 

 Each avicularium has two 

 ' mandibles,' of which one 



is fixed, like the upper iaw 



FIG. 689. A, portion of Bicellana cihata, en- f i i ^i ,-i^ F J 

 larged ; B, one of the ' bird's head ' processes of O a bird, the otner mov - 

 Bugula avicularia, more highly magnified, and able, like its lower jaw ; the 

 seen in the act of grasping another. l atter ig O p en ed 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 n 



i For a more detailed account of the structure and classification of the marine 

 Polyzoa see Professor Van Beneden's ' Recherches sur les Bryozoaires de la cf>te 

 d'Ostende' in Mem. de I'Acad. Boy. de Bruxelles, torn. xvii. ; Mr. G. Busk's 

 Catalogue of the Marine Polyzoa in the Collection of the British Musemn; Mr. 

 Hmcks's British Marine Polyzoa, 1880; and Nitsche,' ' Beitrage zur Kenntiiiss der 

 Bryozoen, m Zeitschnftf. wiss. Zool. Bde. xx. xxi. xxiv. Of the more important 

 recent publications we may note Mr. Busk's Reports on the Polyzoa of the Chall en <,<> 

 voyage ; Mr. Harmer, On the Structure and Development of Loxosoma ' and 'On 

 the Life-history of Pechcelhna,' in vols. xxv. and xxvi. of the Quart. Journ. of 

 Microsc. Sci. ; J. Barrois, Recherches sur TEmbryologie des Bryozoaires,' Lille 1877 

 and other memoirs; W. J. Vigelms, ' Morphologische Untersuchungeii iiber Flustra 

 Membranaceo-truncata,' Biolog. Centralblatt, iii. p. 705 and Bijdranen tot <!< 

 DierJcunde, xi. For a general account see Professor Ray Lankester's article ' Polyzoa ' 

 in the 9th edition of the Encyclopedia Britannica, and Dr. Harmer's work alreadv 

 referred to. 



