GENERAL. INTRODUCTION. 



THEORIES AS TO THE ORGANISATION OF SIPHONOPHOR.E. 



The morphological and physiological peculiarities which distinguish the Siphonophorse 

 from other Acalephse have led, as every zoologist is aware, to very diverse theories as to 

 the real nature and import of their organisation. Two essentially different interpretations 

 still stand in marked antithesis to one another, and these may for the sake of brevity be 

 described as the Poly-organ and Poly-person theories. Of these the poly-organ theory 

 is the older (Eschscholtz 1829, Huxley 1859, P. E. Midler 1871, Metschnikoff 1874). 

 According to this view the adult organism of all Siphonophorae is a simple " Medusa-like 

 animal," which is distinguished from the typical Medusae only in the multiplication 

 and differentiation of its polymorphic organs. More accurately defined, the organism 

 remains an individual Hydromedusoid person (a morphon of the third grade, or a 

 " morphological individual of the third order of individuality"). 



In contrast to this older interpretation is the poly-person theory (Vogt 1848, Leuckart 

 1851, Kolliker 1853, Gegenbaur 1854, Claus 1863, Chun 1882). According to this 

 view the adult Siphbnophoral organism is a colony of animals, composed of many polyp- 

 like individuals, which, according to the laws of the division of labour, have to undergo 

 various modifications, both in the way of specialisation and retrogression. More 

 accurately expressed, it is a " swimming Hydropolyp stock or conn," composed of many 

 polymorphic persons, partly polypoid, partly medusoid (a morphon of the fourth grade, or 

 a "morphological individual of the fourth order of individuality"). 



Since in the course of the last two decennia the ontogeny of Siphonophorae has become 

 better known, and since the latter has in the light of the theory of descent been inter- 

 preted in causal relation to likylogeny, the main antithesis between the two theories has 

 become more marked. According to the poly-organ theory the ontogenetic primitive 

 form of the Siphonophoraj is a simple Hydromedusoid person, and with this is in- 

 volved the phylogenetic corollary that the whole class originally arose from Medusae. 

 According to the poly-person theory, on the other hand, the ontogenetic primitive form 

 is a swimming Hydropolyp stock, and with this is associated the phylogenetic corollary 

 that the whole class sprang from Polyps. 



(ZOOL. CHALL. ESP. — PART LXXVII. 1888.) IThllh 1 



