IV. POLTZOA. 321 



body cavity divided by septa and chsetsd. On the other hand it is leech- 

 like in other features; two suckers and sexual apparatus on the Hiru- 

 dinean pattern. 



Order I. Gnathobdellidae. 



The jawed leeches include Hirudo medicinalis, once extensively used 

 for blood-letting but now little employed. Hcemadipsa includes land 

 leeches, one of the terrors of travelers in the tropics. In Nephelis * the 

 jaws are soft. Macrobdella * includes our largest native species. 



Order II. Rhynchobdellidae. 



Without jaws. The CLEPSINID^E mostly feed on snails and fishes. 

 Clepsine * in our waters. Hcementaria officinalis of Mexico is used for 

 blood-letting ; H. ghiliani of South America is poisonous. The ICHTHYO- 

 BDELLID^E, cylindrical, occur in salt and fresh water, parasitic on fishes. 

 Ichthyobdella,* Pontobdella* marine ; Piseicola, fresh water. 



Class IV. Polyzoa (Bryozoa). 



In external appearance the Polyzoa closely resemble the 

 hydroids, so that the inexperienced have difficulty in distinguishing 

 them. Like them by budding they form colonies which are either 

 gelatinous or calcareous incrusting sheets or assume a more bush- 

 like character. Further they have a crown of ciliated tentacles 

 which can be spread out or quickly retracted. In internal charac- 

 ters the two groups are greatly different. The Polyzoa have a 

 complete alimentary canal, with its three divisions, which is bent 

 upon itself so that the anus lies near the mouth. The central nerv- 

 ous system lies between mouth and anus, and the single pair of 

 nephridia empty by a common opening. Beyond this it is diffi- 

 cult to go, since the two groups of Polyzoa Endoprocta and Ecto- 

 procta differ so widely that one may doubt whether they belong 

 together. The Entoprocta have no coelom, and resemble in this 

 respect the Rotifera; the Ectoprocta are true Coelhelminthes and 

 by way of Phoronis show resemblances to the Sipunculoida and so 

 to the Annelida. 



Sub Class I. Entoprocta. 



The single individuals of the Entoprocta (fig. 294) are shaped 

 like a wine-glass and are placed on stalks which rise from (usually) 

 creeping stolons. The circle of tentacles, arising from the edge 

 of the cup, enclose the peristomial area in which are both mouth 

 and anus, and between these the excretory and reproductive organs 



