THE DISCONTINUOUSLY ZOOIDAL 71 



for the organism of which it is a part, and which itBelf , though 

 only part of an Individual, enjoys a free and independent 

 existence. 



There are, however, Individual types, composed of many 

 separate multicellular organisms, to whom the term " dis- 

 continuously zooidal " is not applicable ; the reason being 

 that in these cases the component organisms are not " zooids," 

 but more complex than these, representing compressed zooid- 

 multiples. For a " zooid " has a narrower definition than 

 a living multicellular organism, part of an Individual. It is 

 truly such an organism, but one built on a special and simple 

 plan, being essentially a little cylindrical tube open at 

 one end, whose walls are composed of two layers of cells. 

 It represents the simplest form of multicellular organism 

 in Nature composed of true tissue. So, from now onwards, 

 " zooid " and " zooidal " will be used to indicate a special 

 form of cellular continuity and a special structural plan, 

 such as exhibited by the freshwater polypite, Hydra, which 

 has been taken as our illustrative type. 



Hydra. 



The common Hydra is a minute little animal which is 

 found with frequency in the water of ditches. In structure 

 it is a tiny hollow cylinder of living tissue of a third or a 

 quarter of an inch in length when its body is extended. At 

 one end the cylindrical body is closed and a circular disc 

 enables the organism to attach itself to twigs or other objects ; 

 but at the other, the distal end, the cavity of the tubular 

 body communicates directly with the surrounding water by 

 an opening which is called the mouth and is provided with 

 marginal tentacles. These are hollow and in communication 

 with the body-cavity or " coelenteron," and being extensile 

 and retractile, it is by their means that the little animal 

 seizes its prey. 



The tissues of the Hydra zooid are on the whole adapted 

 to simple functions, consisting largely of an outer layer of 

 cells called the ectoderm, and an inner layer, the endoderm. 

 Between these lies a very fine structureless layer or membrane 

 called the mesogloea. At the same time there is wonderful 

 cell-specialisation in the presence of stinging thread-cells here 



