Natural History of East Finmark. 571 



briefly to points which afford the chief assistance in the 

 classification of the forms. 



1. The Compensation- Sac. 



The compensation-sac was first observed by JullieUj and 

 has been lately worked out fully by S. F. Harmer, ''On 

 the Structure and Classification of Cheilostomous Polyzoa^'' 

 fProc. Cambridge Phil. Soc. vol. xi. 1900, p. 11). The 

 importance of the compensation-sac is so great that it ranks 

 in classification as dividing the order Cheilostomata into 

 two sections, the one provided with and the other not 

 possessing the compensation-sac. The genera which possess 

 a compensation-sac, and which embrace the greater portion 

 of the Escharine and Lepralian forms, Levinsen ('* Studies 

 on Bryozoa," Vidensk. Medd, fra den Naturh. Foren. i 

 Kjobenhavn, 1902, p. 2, separate copy) proposes to unite 

 under the term Camarostega. 



2. The Front Wall 



Jullien rightly called attention to the importance of taking 

 into consideration the structure of the front wall in the 

 classification. 



3. The Operculum. 



Waters, as long ago as 1878, in his paper "The Use of the 

 Opcrcula in the determination of the Cheilostomatous 

 Bryozoa " (Proc. Manchester Lit. & Phil. Soc. vol. xviii. p. 8), 

 pointed out that the form of the operculum was more reliable 

 in classification than the outline of the oral aperture, since 

 the latter is subject to great modification, Avhile the former 

 is stable. Since that time the operculum has been much 

 studied by Waters, Lorenz, Levinsen, and others. There 

 cannot be a doubt that it is of great value in classification 

 as regards, first, its nature (membranous or calcareous, 

 separable or inseparable) ; second, its form and structure ; 

 and third, the mode of its attachment in the oral opening 

 and the muscular scars which it exhibits. 



4. The so-called ' Rosette-plates ' {or ' Origelles ' of Jullien) 

 and Pore-chambers. 



These have been chiefly studied by Waters, Jullien, and 

 Levinsen. They are destined to play a very important part 

 in classification. The rosette-plates have been studied for a 



