BRYOZOA. 293 



balsam should be tested, and, if it is not hard and brittle, must 

 be reheated. If of the proper hardness, the block is moistened, 

 the slip placed into the excavation, and the superfluous material 

 rubbed away upon the sandstone. When nearly thin enough it 

 is taken out of the block and finished upon the hone. After 

 thoroughly cleaning and drying the section may be covered 

 with a thin sheet of glass, or only a film of balsam, when it is 

 again heated and the air bubbles, if any are present, expelled 

 by faint pressure upon the section with some pointed instru- 

 ment. 



Of course it requires a certain amount of experience and time 

 to make good sections, yet even the beginner ought soon to be 

 able to make from twenty to thirty sections daily, while an 

 expert may increase the number to forty and even fifty. 



For reasons which will become apparent to the reader further 

 along, these sections must be prepared with a knowledge of 

 certain peculiarities which are common to the Bryozoa, other- 

 wise the sections will be misleading. Take for example any 

 ramose or palmate form, and the student will find that the 

 zoarium of such Bryozoa is composed primarily of two distinct 

 zones, an inner or axial region where the zooecia are tubular, 

 with very thin walls, and more or less nearly vertical, and an 

 outer or peripheral region composed of the same tubes bent 

 outwards at varying angles in order to reach the surface. In 

 this outer region the zooecia are supposed to have entered the 

 mature condition, and it is here only that such accessory 

 features as the acanthopores and mesopores are developed. The 

 necessity of two sections, a vertical and a transverse, is at once 

 obvious, but as neither of these sections will give us a cross 

 section of the zooecia in their peripheral region, where the adult 

 and consequently the most important characters are to be 

 found, it is evident that a third section must be prepared, 

 which will enable us to investigate these characters. This sec- 

 tion, which is called ; 'tangential," must divide the zoarium along 

 a plane paralled with the surface, and only a little below it. Of 

 bifoliate forms two tangential sections ought to be made, one 

 passing through the zoarium just below the surface, and the 

 other just above the median lamina. In thin examples of this 



