THE BRYOZOA — RASSLER. 357 



The distinction between the families of Cyclostomata, like other 

 orders of bryozoa, is or should be based on their larval forms, each 

 family being characterized by a special larva. The larvae of the 

 Cyclostomata are very similar to each other and difficult to dis- 

 criminate, but fortunately they show their differences by the evolu- 

 tion of the embryos in ovicells of very different size, form, and posi- 

 tion. The first tube of a zoarium is the ancestrula, and its lower part 

 (pi. 4, fig. 1) is a dilated blisterlike form called the protoecium. It 

 is in the protoecium that the histolysis of the fixed larva and its 

 replacement by the first normal polypide living in the ancestrula 

 occurs. 



Without doubt the same principles of classification applied to 

 the apparently more complicated Cheilostomata, as described on a 

 later page, should be employed in the study of the Cyclostomata ; 

 indeed, a natural classification can be built up only by a study of 

 the physiologic functions of the organs. In the Cheilostomata it 

 will be noted that the form of the aperture and of the operculum, the 

 presence of cardelles, and the modifications of the ovicell are the 

 essential characters of generic and family classification. In the 

 Cyclostomata the aperture is always more or less circular, the oper- 

 culum and cardelles are wanting, leaving the ovicell as the single 

 remaining essential character showing on the zooecia. 



The value of the ovicell in the classification of the Cyclostomata 

 is therefore of utmost importance, but unfortunately its study has 

 been much neglected. Some species of Cyclostomata possibly did 

 not develop ovicells, but the majority of them will after some search 

 undoubted!}' reveal specimens showing this organ. Indeed, one of 

 the most interesting features in the study of the Cyclostomata is 

 the search for ovicelled specimens among the many described species 

 where now no ovicell is known. A beginning toward a natural 

 classification of the Cyclostomata was made by Canu several years 

 ago, and Canu and Bassler in 1920 17 have amplified this subject. 

 The student is referred to their work for more details and references 

 to other researches upon the Cyclostomata. Some of the more com- 

 mon types of ovicell are figured on plate 4. 



In spite of their general simplicity the Cyclostomata exhibit other 

 features which can be used in connection with the ovicell in classifi- 

 cation. For example, in many Cyclostomata there are accessory 

 tubes developed either on the frontal or the dorsal side of the 

 zoarium. These are zooecia, closed or open, which appear to be 

 without a polypide. Thin sections of the zoarium are frequently 

 necessary to determine the nature of such accessory tubes. The 



17 1920. Canu and Bassler. North American Early Tertiary Bryozoa. Bulletin 106, 

 U. S. National Museum. 



