2 THE VOYAGE OF H.M.S. CHALLENGER. 



According to the present opinion of most zoologists, which we also share, the low 

 sessile Hydropolyp form is the more primitive ; the higher Hydromedusa form has 

 been secondarily developed at a much later stage, and that by adaptation to a free- 

 swimming mode of life. In this way has the characteristic swimming organ of the 

 Medusas arisen, namely, the umbrella, with its radially constructed gelatinous disc, which 

 was entirely absent in the pimitive ancestral forms — the Polyps. The most important 

 structure of the latter, however, the gastral tube (inherited from the Gastraea), has been 

 transmitted to the Medusas, and has become the " manubrium," in the wall of which the 

 generative products are developed. 



If we apply this fundamental and firmly based conclusion to the two theories of 

 Siphonophore organisation, the following is evidently the antithesis in regard to the 

 question of origin. According to the poly-organ theory the primitive form of 

 Sipkonophorae was a simple Medusa and already possessed an umbrella ; from this 

 established swimming organ the various locomotor organs of the Siphonophorae (swimming- 

 bells and air-chambers) are derived by multiplication and modification. According to 

 the poly-person theory, on the other hand, the primitive form of the Siphonophorse 

 was a Hydropolyp colony, and possessed no umbrella ; the locomotor organs which are 

 present are therefore new structures, not to be derived from any pre-existing 

 swimming organ of the primitive form. And this leads to a weighty contrast in regard 

 to the Medusiform larvae, which arise directly from the gastrulae of Siphonophoras. 

 According to the poly-organ theory, such a larva possesses essentially the morphological 

 value of a simple Medusoid person, and as the hereditary repetition of the original 

 primitive form has the greatest palingenetic importance. According to the poly-person 

 theory, on the contrary, it possesses no such importance ; it is merely of subordinate 

 kenogenetic value, and is to be regarded as a peculiarly modified Hydroid polyp. 



Both these opposing theories have been for forty years supported with much 

 acuteness by distinguished zoologists, but yet without decisive conclusion ; both are in 

 fact partially justified ; both contain a mixture of truth and error. According to my 

 own opinion, which is based on an extensive comparative investigation of the entire 

 class, and on numerous new facts discovered in the process, the truth lies midway 

 between the two interpretations. The poly-organ theory is right in starting in its 

 whole interpretation and rationale of the Siphonophorse from a Hydromedusoid type, 

 in regarding the primary medusiform larva as 'palingenetic, and further in supposing 

 an extensive multiplication and dislocation of the several Medusa organs. It is wrong, 

 however, in attributing to the fully developed Siphonophoral corm the value only of a 

 person, and in regarding the persons which compose the stock purely as organs in the 

 morphological sense. The poly -person theory, on the other hand, is right in explaining 

 the fully developed Siphonophore as a corm (colony or stock), composed of many poly- 

 morphic persons. It goes, however, much too far, and is in error when it seeks to 



