INTRODUCTORY OBSERVATIONS. 



11 



Fiar. 4. 



The surface of the cell varies infinitely in appearance ; it may be either smootli tuul 

 entire, spinous or granulous ; perforated with minute pores, or cribriform with larger 

 openings ; reticulate or ribbed, &c., all of which conditions, with certain precautions, afford 

 excellent diagnostic characters. The margin of the orifice, sometimes 

 termed the "peristome" may be simple or thickened, unarmed or beset 

 with erect " marginal spines," Avhich again may be cither rigid or 

 articulated at the base, simple or branched. 



The Cheilostomata, moreover, as a class are especially distin- 

 guished by the position and form of an organ termed the " ovicell," 

 and by the possession of certain external agents of offence and 

 defence, termed " avicularia " and " vibracula." 



The ovicell is not found in every species, and when present is 

 in some cases so deeply immersed as to be inconspicuous. In a 

 great many species, however, it is apparent in the form of a more or 

 less rounded eminence situated above or behind the cell, as shown 

 in fig. 4. The cavity of the organ is continuous with the perivisceral 

 space, through a passage situated at the upper and back part of the cell, and through which 

 it would appear the ova are conveyed as into a sort of marsupial pouch. This organ is 



o. Ovicell. 



c. Avicularium. 



Fig. 5. 



wanting in the Cyclostomata, in which its functions are apparently 

 supplied by a dilatation of the body of the cell itself. 



The other class of organs above referred to is also peculiar to the 

 Cheilostomata, though wanting in many of them. For a detailed 

 account of these interesting appendages the reader may refer to 

 some " Observations " respecting them contained in the British 

 Museum Catalogue of Marine Polyzoa, or to a paper on the subject 

 in the ' Transactions of the Microscopical Society.'^ It will be suffi- 

 cient here to remark that the avicularia are so termed from the strong 

 resemblance the earliest and best known form of them presents to a 

 bird's head (fig. 6). They may be subdivided, for the purpose of syste- 

 matic description, into three sorts — the sessile (fig. 5), the immersed 

 (fig. 4 c), and the pedunculate (fig. G). But whatever its diversity of form, an avicularium 



always consists of two parts, viz., a moveable mandible, and a cup 

 furnished with a horny beak, with which the point of the mandi- 

 ble is capable of being brought into opposition. The cup contains 

 muscles properly arranged for the elevation and depression of the 

 mandible, and other parts which need not here be particularised. 

 The vibracula are organs very similarly constructed, but 

 having, instead of a mandible, an elongated seta, capable of ex- 

 Peduricuiaie avicularium. tcusive movcmcut principally in one plane. 



Sessile avicularium. 



'Quart. Journ. Microscopical Science,' vol. ii ('Transactions,' p. 26), 18J4. 



