350 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



Various other preparations necessary in the study of certain groups 

 of bryozoa are described under the discussion of these groups. 



In the study of the bryozoa two bibliographies are useful, one by 

 Miss Jelly on the recent species 8 and the second by Nickles and 

 Bassler, 9 which, in addition to the citations of all North American 

 fossil species, includes a complete list of the literature up to the date 

 of publication, classified in various ways for easy reference. 



TYPES OF BRYOZOA. 



The zooecial structural features and methods of study of the 

 bryozoa differ so decidedly for the various divisions that it is pref- 

 erable to consider each one separately. 



Subclass ENTOPROCTA. 



Of the two very unequal major divisions of the bryozoa, the En- 

 toprocta, characterized, as indicated in the name {endon= within, 

 proktos=amis) , by the position of the anal opening within the row 

 of tentacles, is especially interesting in that the comparatively few 

 species classified here probably represent the most primitive expres- 

 sion of the class. In this subclass the tentacles during retraction of 

 the polypide are infolded into a vestibule which can be closed by a 

 sphincter. Definite excretory organs are present as are also reproduc- 

 tive organs which have ducts leading to the vestibule. The different 

 zooids or individuals formed by budding, are further marked by 

 their isolation from each other. In this respect the subclass differs 

 from almost all other brj^ozoa, as the rule is for adjacent zooecia to be 

 in contact. In Loxosoma, a typical entoproctous genus, colonies even 

 are not formed as each zooid leads an independent existence. 



As shown in figure 3, these bryozoa grow from a thread-like stolon 

 emitting cylindrical stalks each of which expands into the body of a 

 zooid. The calyxlike zooids are lost from time to time and then the 

 end of the stalk generates another polypide bud which matures into 

 a new calyx. In no case is the body wall calcified so that the En- 

 toprocta is not represented in the fossil state. 



Loxosoma and Pedicellina represented in figure 3, and TJrnatella 

 ore the best known genera. TJrnatella is a beautiful form found at 

 present only in fresh water in the United States. In this genus the 

 calyx surmounts a segmented stalk and the stalks arise quite regu- 

 b.rly in pairs from a common base. For a more detailed account of 

 the Entoprocta the reader is referred to Ehler's work of 1890. 10 



8 1889. Jelly, E. C. Synonymic Catalogue of Recent Marino Bryozoa. 



9 1900. Nickles and Bassler. Synopsis of American fossil bryozoa. Bull. 173, U. S. 

 Geol. Surv. 



10 1890. Ehlers, Zur Kenntniss der Pedicellineen. Abhandlungen der physical. Klasse 

 c!er Koniglichen Gcsellschaft der Wissenschaften zu Gottingon, XXXVI, pp. 1-200, pis. 

 1-5. 



