MOLLUSCOIDA FOLYZOA. 119 



by one extremity to some solid object. They are so very 

 minute that 1,800 cells, each containing a separate animal, 

 Lave been found in one square inch of surface. 



To the outside of some of the cells certain organs are 

 often attached which bear a considerable resemblance to 

 the beak of a bird. They have been called avicularia 

 (Lat. avicula, a little bird), or " bird's head processes." 

 The beak consists of two mandibles, the lower of which 

 only is movable. This organ is quite independent of the 

 animal to which it is attached, for it continues its snapping 

 movements long after the death of the zooid. The use 

 of these curious appendages is not known. They are 

 supposed to bear some analogy to the pedicellarise of the 

 sea-urchins and star-fishes. 



A colony f these animals is called a polyzoarium, 

 each individual animal a polypide. The term zooid is 

 used both in Hydrozoa and Polyzoa, as a common designa- 

 tion for the individual organisms that make up a colony 

 which has been produced by gemmation or budding. 



172. Skeleton. Each polypide is enclosed in a double- 

 walled cell. The outer layer is either chitinous or cal- 

 careous, and is called the ectocyst (Gr. ektos, without ; 

 hustis, a bladder). The inner lining of the ectocyst is 

 membranous, and is called the endocyst (Gr. endon, 

 within; kustis, a bladder). There are two openings in this 

 membrane, one for the mouth and the other for the anus. 

 These openings are generally placed near one another. 



173. Digestion. The mouth is surrounded by a num- 

 ber of hollow tentacles which are provided with cilia. In 

 the marine species, these tentacles are arranged in the 

 form of a circle ; in those that live in fresh water, they 

 assume the shape of a crescent or horse-shoe. The cilia, 

 by their perpetual movements, produce little whirlpools in 

 the water which supply the animals with those nutritious 

 particles on which they subsist. 



When the animal is alarmed, it is able by a special set 

 of muscles, connecting the lower wall of the cell with the 

 gullet, to retract the front part of the alimentary tube 



