4 BULLETIN" 16 5, UNITED STATES NATIONAL MUSEUM 



We have chosen 2 millimeters square (4 square millimeters) as a 

 convenient unit, because this usually permits estimates that are 

 neither too small nor too large. For the giant species the numbers 

 are small, without, however, losing their characteristic value. For 

 species of the Membranipore type, it is necessary to count the cells 

 or at least the opesia, which makes an estimate a little less exact. 

 For species of the Escharian type we count the number of apertures 

 visible in this same space. In general the numbers observed are 

 between 10 and 30. For the small species the number becomes too 

 large and cannot be easily and correctly estimated. For such cases 

 we have chosen another unit, 1 square millimeter, which furnishes 

 a more convenient number ranging between 5 and 15. All the 

 Retepores and many of the Adeonidae can be calculated only by 

 the square millimeter- In the Onychocellidae and all species with 

 large interzooecial avicularia we count the latter as well as the 

 onychocellaria as true cells- In the Celleporidae it is necessary to 

 compute the oriented and the cumulated cells separately. 



Thus, the number of cells on a given surface varies according to 

 the size of the cells. For example, this number is larger in the 

 neighborhood of the ancestrula, where the zooecia are much smaller ; 

 it is necessary then to take the measurements on the zoarial margin, 

 where they are of normal size. Again, irregularities of gemmation, 

 whatever the cause, change the number and measurements taken 

 on unoriented cells. Precision in choosing the right place is there- 

 fore indispensable. In the Berenicea types this method of evalua- 

 tion of the number of orifices on a given surface is preferable to 

 the system of micrometric measurements. 



TERMINOLOGY 



The terminology of the Bryozoa is so special that for the sake of 

 completeness we have introduced the following definitions : 



Ancestrula: First cell (derived from the larva) of a colony. 



Anter: Superior distal portion of the apertura. 



Apertura : Orifice of the cells through which the animals extend their tenta- 

 cles ; it is closed by an operculum in the Cheilostomata. 



Ascopore: Frontal pore serving as the orifice to the compensatrix. 



Avicularium : Small cell without polypide containing muscles producing move- 

 ment in a mandible, always in the same direction. 



Cardelles and Lyrule : Small lateral points placed in the apertura and serving 

 as pivots to the operculum ; the lyrule, placed in the middle, limits the 

 movement of the operculum. 



Compensatrix : Small membranous sac placed in the cell, which becomes filled 

 with water when the tentacles are extruded. It opens exteriorly in the 

 proximal portion (poster) of the aperture or by a special pore (ascopore). 



Costules : Small radiating ridges forming the frontal of the cribrimorph cells ; 

 they originate from primitive marginal spines. 



