40 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
large divisions under which the Bryozoa are classed (Entoprocta and Ectoprocta.) 
are based upon the position of the anal opening. In most cases the anal opening 
is situated without the row of tentacles (Ectoprocta) rarely it is placed within 
this row (Entoprocta). A heart and vascular system are wanting, but there are 
numerous leucocytes floating in the general cavity. A nervous ganglion is present 
between the mouth and anus and sends delicate nerve filaments to the tentacles 
and esophagus. The upper part of the sac is generally flexible and can be invagi- 
nated through the action of numerous longitudinal and transverse muscles which 
traverse the fluid-filled visceral cavity. 
The reproductive organs are developed in various parts of the body cavity, 
although the spermatozoa occur usually in the lower and the ova in the upper part. 
The ova may be developed in a special receptacle, in an inflation of the surface 
or in a modified zooecium. The general term ooecium or ovicell is applied to all of 
these structures. 
Many bryozoans exhibit, attached to the zooecium, organs resembling a bird’s 
head, termed avicularia, and other bristle-like appendages named vibracula. The 
jaws of the avicularia open and close with a snapping motion which has given 
rise to the probably erroneous idea that they are organs of defense. The structure 
of these two organs is described in more detail in the consideration of the 
cheilostomatous bryozoa. Both the avicularia and vibracula are incapable of 
preservation in the fossil state, but their former presence is indicated by the pore- 
like excavations in which they lodged. 
The extended polypide is withdrawn into the zooecium by the contraction of 
retractor muscles attached to the tentacular crown. In the bryozoa with flexible 
zooecia the contraction of the body walls by parietal muscles produces protrusion 
of the polypide, but in the rigid calcareous zoaria the means for protrusion are 
more complicated (see p. 59). 
CLASSIFICATION. 
The Bryozoa and the Brachiopoda are considered as constituting the phyllum 
Molluscoidea, although some authors believe there is no relationship between them 
and regard the Bryozoa as representing a distinct phyllum. The two large sub- 
divisions of the Bryozoa, Ectoprocta, and Entoprocta, based upon the position of 
the anus with reference to the tentacles have been mentioned before. These 
subclasses differ widely from each other in many respects and here again some 
authors believe they are not even distantly related. However, the great majority of 
these animals belong to the Ectoprocta and under this to the superorder Gymnolae- 
mata. Five orders of Gymnolaemata are known, of which the Cheilostomata is 
perhaps the largest in number of species. The relations of these various classi- 
ficatory terms are expressed in the following table: 
