THE BRYOZOA BASSLER. 



371 



liferous side down, and then rubbing away the superfluous material 

 until the inner side of the calcified frontal wall is revealed when the 

 true nature of the frontal unchanged by any external influence may 

 be found. 



The preparation of the operculum, which remains only on recent 

 forms of course, is another important and quite simple operation. 

 The simplest way to prepare slides for viewing under the microscope 

 is to scrape ofF a few zooecia Avith the operculum in place, crush these 

 carefully in a drop of water on the slide and after drying add 

 Canada balsam and a cover glass. Some of the opercula will be 



-perlstomice 



peristome 



r open operculum 



membraneous portion, 

 of the spiramen. 



-anter 

 closed operculum' 



-tentacular sheath 

 dorsal zooecial wall 

 polypide 



' splra,men 



posten 

 compensatrix 



frontal zooec- 

 ial wall 



retractor muscles 

 of the polypide 



Pig. 13. — Structure of the Ascophora. Tesaardoma gracile Sars. A recent species of the 

 Ascophora showing a zoarial branch, X 60, and a drawing of a longitudinal section 

 through a single zooecium with the various parts of the zooecium and polypide indi- 

 cated. (After Hincks and Jullien.) 



broken by this crude method, but enough perfect specimens will re- 

 main to make the saving of time worth while. 



Formation of the zoarial skeleton. — The living tissue of the bryo- 

 zoan giving rise by its differentiation to all the various organs is a 

 delicate epithelial membrane, the endocyst, lining the interior of the 

 skeletal parts. The first differentiation of the endocyst is the ecto- 

 cyst, a thin outer covering membrane which has no secreting power. 

 Next, the endocyst secretes the mesenchyme, which in turn gives origin 

 to the polypide and other portions of the organism. 30 The calcare- 

 ous or chitinous secretion forming the zoarial skeleton occurs be- 



M 1900. Calvet, L. Contributions a l'historie des bryozoaires ectoproctes marins. Tra- 

 vaux de l'institut Zoologie de l'Universite de Montpellier, new ser., Memoire No. S. 



