NORTH AMERICAN EARLY TERTIARY BRYOZOA. 



651 



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OVICELLS. 



The ovicells of the cyclostomatous bryozoa are usually rare. Moreover, they 

 are very fragile and resist fossilization very poorly. Their form and nature give 

 the essential characters of an entirely nat- 

 ural classification. This new conception 

 has obliged us to greatly modify and 

 correct the zoarial classification hitherto 

 generally recognized. 



The ovicells allow the larva to escape 

 by an orifice called the oeciopore, which 

 is surrounded by a more or less salient 

 collar called the oeciostome. In another 

 group the escape of the larva occurs by 

 the rupture of the walls and there is no 

 oeciostome. 



The studies concerning the ovicells 

 of the fossil bryozoa are still very incom- 

 plete; it is often impossible to discover 

 the oeciostome on a single specimen and 

 consequently to find a good generic char- 

 acter immediately. The special mode of 

 formation of the larva is the cause of 

 this rarity of the ovicells. The fertile 

 egg is developed by successive segmenta- 

 tion into a large primary embryo occupy- 

 ing almost all the ovicell. In the latter, 

 by fissiparity secondary embrj'oes are de- 

 tached which are evolved in the ordinary 

 manner and escape by the oeciostome. In 

 this manner 150 larvae may be sent out 

 from a single ovicell (fig. 214). 



Figiire 226 gives the form of the 

 ovicell in each family discussed in this 

 volume. 



n.4. 



FIG. 214. Fissiparity of primary embryo. 



Section of a mature ovicell of Crisia ebunca 

 Linnaeus, 1758, X 200, showing the primary em- 

 bryo (prim, emb) giving off buds (sec. emb) 

 and also larvae (lar). (After Robertson.) 



SYSTEMATIC DESCRIPTION OF THE CYCLOSTOMATA. 



Our studies have caused us to reject for the present all of the former major 

 classifications of the Cyclostomata and to retain simply for convenience the two 

 larger divisions of the Inovicellata and Ovicellata. Under the first of these terms 

 we recognize two subdivisions (a) the typical Cyclostomata, or those with club- 

 shaped tubes and (&) trepostomatous-like Cyclostomata, or those with cylindrical 

 tubes. 



