372 



ANNUAL REPORT SMITHSONIAN - INSTITUTION, 1920. 



tween the ectocyst and endocyst and with all of its variations in struc- 

 ture and accompanying organs, such as vibracula and avicularia, is a 

 result of the activity of the endocystal buds. The walls of the zoarial 

 skeleton may consist simply of a smooth thin calcareous deposit, the 

 olocyst, or above this may be secreted a second very porous layer, the 

 tremocyst, intimately joined with the olocyst, although sometimes 

 clearly detachable. A third layer, the pleurocyst, consisting of a 

 granular deposit with lateral punctations, may also occur. The pores 

 of these several layers are traversed by mesenchymatous fibers which 

 likewise pass from zooecium to zooecium through the lateral walls 

 by small pores called septulae. These may be uniporous or multi- 



FlG 



Septulae and diatellae. 



1—3. Septulae or parietal pores through which the mesenchymatous fibers pass 

 from one zooecium to another. 1. Edge view of a zooecium of Cheilopora 

 slncera Smitt with multiporous septulae developed ( = rosette plates of 

 authors). 2. A greatly enlarged view of a multiporous septula illustrating 

 details of the structure. .".. Edge view, X23, of portions of two zooecia of 

 Hippopodina fegcensis Busk showing uniporous septulae through the lateral 

 walls. 



4—8. Dletellac or pore chambers. Views of various species of Cheilostomata 

 magnified, as seen from the basal side and illustrating the variation in as- 

 pect of the diatellae. 4. Peristomella prestans Hincks. 5. Elllsina levata 

 Hincks. G. Callopora lincata Linnaeus. 7. Cauloramphus spinifer Norman. 

 8. Trypostcga venusta Norman. 



porous (text fig. 14), but before reaching the septulae the mesen- 

 chymatous fibers traverse small lateral chambers in the proximal 

 part of the zooecium, called dietellae or pore chambers. 



The discrimination of the characteristics of these various zooe- 

 cial skeletal features is important in the determination of genera and 

 species, and so it is necessary in study that the following preparations 

 be made. First, thin sections of the wall, particular the frontal, are 

 needed to illustrate the characters of the three layers, olocyst, tremo- 

 cyst, and pleurocyst. Second, the frontal must be abraded away to 

 show the occurrence of such structures as dietellae. This abrasion is 

 effected by mounting the specimen, frontal side up, in Canada balsam 



