POLYZOA 191 



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 individuals of a colony may 

 communicate 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 zooecium may be smooth or punctate, or ornamented 

 with spines, granules, or ribs. 



The Polyzoa are divided into two classes, (1) the 

 Ectoprocta, (2) the Entoprocta. The Ectoprocta only are 

 found fossil. 



CLASS I. ECTOPROCTA 



The anal aperture is not situated within the area of 

 the lophophore. There are two orders, (1) the Phylacto- 

 lsema, (2) the Gymnolsema. 



ORDER I. PHYLACTOL^MA 



The lophophore is horse-shoe-shaped. There is a 

 tongue-shaped lip in front of the mouth, known as the 

 epistome. The forms included in this order are found 

 only in fresh-water and do not occur fossil. 



ORDER II. GYMNOL^MA 

 The lophophore is circular, and there is no epistome. 

 There are five sub-orders, (1) Cyclostomata, (2) Treposto- 

 mata, (3) Cryptostomata, (4) Cheilostomata, (5) Ctenosto- 

 mata. The last is not known in the fossil state; the 

 second and third are extinct. 



SUB-ORDER I. CYCLOSTOMATA 

 The zooecia are calcareous and tubular, and seldom 

 divided by transverse partitions ; as a rule all are of one 



