298 COELENTERATA HYDROZOA chap. 



animal's body, it iniglit be tliought obvious that the plienomeiion 

 observed in the Medusae of Syncoryne is a case of a simple 

 repetition of the parts of an individual ; Ijut tlie power tliat 

 the group of manubria possesses of leading an independent 

 existence renders its interpretation as a group of organs a 

 matter of some i)iconvenience. If we can conceive tlie idea 

 tliat an organ may become detached and lead an independent 

 existence, there is no reason why we should not regard the 

 Medusa itself of Syncoryne as an organ, and we should be 

 driven to the paradoxical conclusion that, as regards several 

 genera and families of Hydrozoa, we know nothing at present 

 of the individuals, but only of their free-swimming organs, and 

 tliat in others the individual has degenerated, although one of 

 its organs remains. 



There is, however, no convincing argument to support either 

 the conception that the Siphonoplioran body is a colony of 

 individuals, or that it is an individual with disjointed organs. 

 These two conceptions are sometimes called the " Poly-person " 

 and " Poly-organ " theories respectively. The difficulty is caused 

 by the impossibility of giving any satisfactory definition in the 

 case of the Hydrozoa of the biological terms " organ " and 

 " individual." In the higher animals, where the correlation of 

 parts is far more complex and essential than it is in Coelenterata, 

 a defined limit to the scope of these terms can be laid down, 

 but in the lower animals the conception of what is termed an 

 organ merges into that which is called an individual, and no 

 definite Ijoundary line between the two exists in Nature. The 

 difticulty is therefore a permanent one, and, in using the expression 

 " colony " for the Siphonophoran body, it must be understood 

 that it is used for convenience' sake rather than because it 

 represents the only correct conception of the organisation of 

 these remarkable Coelenterates. 



Kegarding the Siphonophora as polymorphic colonies, then, 

 the following forms of zooids may be found. 



Nectocalyces.' — The nectocalyces are in tlie form of the 

 umbrella of a medusa attached to the stolon of tlie colony by 

 the aboral pole. They are provided with a velum and, usually, 

 four radial canals and a circular canal. There is no manubrium, 

 and the marginal tentacles and sense-organs are rudimentary or 

 absent. There may be one or more nectocalyces in each colony. 



