102 INTRODUCTION TO POLYZOA. 



wonderful plasticity of nature, as evidenced in 

 this example of the specialization of the 

 individual. 



Epistome. There is one organ which 

 has hitherto not been described, since the 

 majority' of Polyzoa are distinguished by its 

 absence. This organ is the " Epistome," a 

 hollow lobe of the body-wall, overhanging the 

 mouth upon the anal side. It is ciliated and 

 appears in one marine species to aid the 

 animal in ascending its tube, thus performing 

 a function similar to that of the molluscan 

 foot, of which it is regarded the precursor. 

 The group of the Polyzoa in which the 

 epistome occurs is called the Phylactolaemata. 



Classification. The three most 

 important distinctions in the Polyzoa are : 



1. Whether the anus lies outside 

 (Ectoprocta) the tentacle stage, or within it 

 (Endoprocta). 



2. Whether the organ known as the 

 Epistome be absent (GYMNOL^MATA*), or 



present (PHYLACTOLJEMATAf). 



*Signifies "naked-throat," i.e., without epistome. 

 t Means "throat-guarded," i.e,, with epistome. 



