54 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



Diplodidymia Eeuss. 1867) ; but often also the female zooecia have a special form 

 suitable for their function (=gonoecia of the Adeonidae and Catenicellidae) . 



In actual experience it is necessary to have a very large number of specimens 

 to establish that any fossil species entirely lacks ovicells, for these organs are very 

 fragile. 



Nature of the ovicells. Most of the Cheilostomata have some visible ovicells. 

 Their nature is not always peculiar to a particular larval system. Nevertheless 

 the endotoichal ovicell appears peculiar to the Cellariidae, the recumbent ovicell 

 to the larvae of the Phylactellidae and the Conescharellidae and the peristomial 

 ovicell to the larvae of the Tubucellariidae. In all other cases many very different 

 systems of larvae may be developed in ovicells apparently identical ; these latter are 

 endozooecial or hyperstomial. 



The endozooecial ovicells are formed by the distal portion of the zooecium; 

 in certain genera they are separated from the zooecium by a fold of the wall 

 (Flustridae, Hippopodinidae, Farciminariidae, Onychocellidae, etc.). 



The hyperstomial ovicells are placed on the distal zooecium. A very impor- 

 tant group among them has their orifices closed by a distinct operculum formed 

 for the most part by the ectocyst itself; this is the group of the Aneucleithriens. 

 In practice, on the fossil forms, such ovicells are recognized by their frontal which 

 does not hide the apertura ; seen in profile, their orifice is perceptibly perpendicular 

 to the apertura. 



A second group has the orifice always closed by the operculum; this is the 

 group of the Cleithriens (Micropora, etc.). Practically, on the fossils these are 

 recognized as follows: First, their frontal partially covers the apertura; second, 

 seen in profile, their orifice is oblique to the apertura ; third, the common orifice 

 is identical and of the same size in all the ovicelled zooecia. 



The third group, that of the Subclcithriens, is intermediate between the two 

 preceding ; the operculum closes the ovicell only in opening. This opercular function 

 is certainly intended to protect the passage of the eggs. On the specimens deprived 

 of their opercula. the distinction is very subtle, especially as the closing is not 

 always perfect. Practically on the fossils, subcleithrien ovicells are recognized by 

 the following points: First, their frontal partially covers the apertura; second, 

 seen in profile their orifice is oblique to the apertura ; third, the common orifice is 

 identical with the true orifice; fourth, the common orifice is identical with the 

 apertura of the unovicelled zooecia. 



When the operculum is fragile the ovicell is more or less deeply imbedded in 

 the distal zooecium ; the apertura is oblique ; the ovicell opens into a small, free 

 space called the loc.eUa (Peristomellse). 



Again, when the operculum is fragile the frontal becomes much thickened 

 and the ovicell imbedded in the distal zooecium opens into the peristomie thus 

 formed. The apertura, more or less visible and oblique, is placed at the base of 

 this peristomie (Smittinidae). The exterior orifice is therefore never closed by the 

 operculum and it is very irregular. 



