vin POLYZOA 181 



occurring in lakes, streams, ponds, and even in puddles the water of 

 which is rendered foul and opaque by mud and sewage. Frequently the 

 water in which they live is dried up, and the thick-shelled winter eggs 

 may then be widely dispersed by wind. It is even stated that the adult 

 animals may survive prolonged desiccation and resume active life when 

 again placed in water. Many forms cling to the bodies of higher 

 animals in order to obtain a share of their food, thus leading a sort of 

 commensal existence. Others go a step further and become true 

 external or internal parasites. 



2, THE POLYZOA 



The Polyzoa are an extensive class of animals, for the most part 

 marine, which, from the general form that they assume, are readily 

 mistaken for hydroid zoophytes (Hydrozoa, p. 92). They occur as 

 fixed colonies, the form of which varies greatly, supported by an exo- 

 skeleton which is sometimes gelatinous, sometimes chitinoid, sometimes 

 calcareous. Most usually the colony is a branching, plant -like structure, 

 though it may assume other forms. The whole consists essentially of a 

 number of minute chambers, or zocecia as they are termed, each formed 

 by the exoskeleton of one of the zooids. Each zooecium (Figs. 95 and 

 96) has an aperture, sometimes capable of being closed by a lid or 

 operctilum, through which the oral extremity of the zooid can 

 be protruded. At this protrusible oral end of the zooid is a circular 

 or horse-shoe-shaped ridge or lophophore bearing a number of simple, 

 slender, ciliated tentacles (tent. ). In many Polyzoa the colony bears a 

 series of remarkable appendages, the avicularia, of the nature of modi- 

 fied zooids. A typical avicularium (Fig. 95, avic.) has very much the 

 appearance of a bird's head supported on a very short stalk, with a 

 movable part, representing the lower jaw, which becomes separated from 

 or approximated to the part representing the upper jaw in a manner which 

 closely resembles the movements of opening and closing of the bird's 

 mouth. These are probably defensive organs. The mouth (mo.) is a large 

 aperture in the middle of the oral extremity within the lophophore : the 

 anus is situated near it, but outside the lophophore. The digestive 

 canal is a U-shaped tube, divided into pharynx (ph.), stomach (stom.), 

 and intestine (int. ), suspended within a wide body-cavity. There is no 



