NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
651 
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 embryoes 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). 
Figure 226 gives the form of the 
ovicell in each family discussed in this 
volume. 
Fig. 214. — Fissiparity of primary embryo. 
Section of a mature ovicell of Orisia eburnea 
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 (5) trepostomatous-like Cyclostomata, or those with cylindrical 
tubes. 
