104 POLYZOA. 



fig. 28 J5) the zocecium is tube-like, the aperture is at the 

 end and is of the same diameter, or nearly so, as the rest 

 of the tube. In others (the Cheilostomata, fig. 28 A) 

 the zocecium is more or less oval, the aperture (m) is con- 

 tracted and is not terminal, but is situated in front near 

 the anterior end, and is provided with a movable lid or 

 operculum. In many of the Cheilostomata, there is at 

 the anterior end of the zocecium, above the aperture, a 

 projecting chamber (o), into which the ova pass, termed 

 the ooecium. Many of the Cheilostomata also possess 

 appendages to the zotecia, termed avicularia (fig. 27 B) 

 and vibracula, but these are rarely preserved fossil. The 

 avicularium may be sessile or placed on a peduncle (p), 

 and in the more specialized forms, has somewhat the 

 appearance of a bird's head, consisting of a chamber 

 (C) produced into a beak and provided with a mandible 

 (md) which is kept constantly snapping by means of 

 muscles in the chamber. The vibraculum consists of 

 a long seta kept in motion by means of muscles at its 

 base. The individual polypides of a colony may commu- 

 nicate with one another, either directly, or by means of 

 communication-plates ; these are portions of the zooecium 

 which are thinner and perforated. The surface of the 

 zocecium may be smooth or punctate, or ornamented with 

 spines, granules, or ribs. 



The Polyzoa are divided into three sub-classes, (1) 

 Ectoprocta, (2) Entoprocta, (3) Aspidophora. The Ecto- 

 procta are the only forms found fossil. 



SUB-CLASS. ECTOPKOCTA. 



The anal aperture is not placed within the area of the 

 lophophore. There are two orders, (1) Phylactoloema, 

 (2) Gymnolsema. 



