CLASSIFICATION 29 



a cross-section to demonstrate these relationships, is a matter of opinion, and that there must be 

 consistency in the choice. Even if the phylogeny of a number of species were known, it might be quite 

 correct to adopt any one of a number of generic groupings, according as to where in the past the cross- 

 section was taken. Judge, then, of the systematists' difficulty when the phylogeny is not known. 



THE PIONEER WORK OF HENRY B. BIGELOW 

 Many additions have been made to our knowledge of the Siphonophora since Bigelow, forty years 

 ago, published his famous 'Albatross' Report, which has been a key-work on the group ever since. 

 It was a study of this delightful work that first lured me to examine these animals and to build 

 up an enormous collection at the British Museum (Nat. Hist.). Hardly a day passes when it is not 

 necessary to refer to the work again. If in my systematic notes I often appear to criticize this classic, 

 it is only with the desire to build firmly on sure foundations. Bigelow's figures of siphonophore 

 structure in that, and subsequent reports, are unrivalled. Only he and Dr Mary Sears know how 

 painstakingly he worked upon them. To orientate these animals, whose density is so nearly that of the 

 media in which they have to be examined, and to hold them in position without movement for drawing 

 is a major difficulty in the study of the group. 



GENERAL CONCLUSIONS 

 To sum up my tentative conclusions on the phylogeny of Siphonophora — reached, it has to be 

 admitted, without full consideration of either Rhodaliids or Apolemids — there is evidence that leads 

 to the deduction that siphonophore ancestors, like the primitive Trachylines, had an adult medusoid 

 stage (medusoid gonophores, nectophores), as well as a larval polypoid one (present-day larvae, 

 gastrozooids, palpons). In the course of evolution, and after the adoption of the budding process, 

 the two phases instead of following one another, as in scyphomedusan and trachyline metamorphosis, 

 appeared simultaneously in one and the same bud-colony (gastrozooids and gonophores) as, for 

 example, in Agalma. 



After the development of an aboral organ as a gas-secreting gland (pneumatophore), this gland 

 was once again lost (Calycophorae), when medusoid gonophore-buds were developed precociously 

 in the larval stage as swimming organs. The precociously developed and functionally altered 

 (gonophore -> nectophore) larval organs were then carried into adult life and improved upon as adult 

 swimming organs (nectophores). But some of the larvae of these bud-colonies never adopted the device 

 of using a precociously developed gonophore as a swimming organ and so the adults (e.g. Cystonects, 

 and Athorybia rosacea) do not swim to-day by jet propulsion. 



It should here be emphasized that this interpretation of a very complicated evolutionary story bears 

 no resemblance to the old classical ' medusome ' theory of Haeckel's, which was exploded by Garstang. 

 It follows that acceptance of the conclusions set forth above is compatible with finding in Siphonophora 

 much evidence of neoteny (Physalia, Nectopyramis diomedeae, Sphaeronectes, Athorybia). This phe- 

 nomenon probably affords the explanation of the existence of both short-stemmed forms (brachysteles) 

 and long-stemmed forms (macrosteles) among the Physonectae, the macrosteles having increased in 

 length and the brachysteles having failed to lengthen in one or two directions. There seems to be every 

 chance that in the future other unspecialized larvae will evolve on new neotenous lines. 



We may, perhaps, in some ways compare the story of the siphonophore nectophore with that of the 

 tail of the Ascidian tadpole, which appears to have been developed as a larval organ, is lost at meta- 

 morphosis by Ascidians, but is retained and developed in the adult stage of other Chordata. In the 

 siphonophore story, of course, the organ (gonophore) adapted specially for larval use (as a nectophore) 

 was originally an adult ' organ ', or rather the adult (medusa) phase itself, produced by budding, and 



