2 8 DISCOVERY REPORTS 



the Cystonectae than that suggested by the larvae of Physonectae or of Calycophorae, namely at 



an ancestral stage in which nectophores had not developed (Text-fig. 5 A). 



For the purpose of this report I have placed Athorybia, Melophysa and Physophora at the head of the 

 list of Physonectae because they are probably neotenous forms, but the relationships between the 

 others are not understood. In the list of Calycophorae, about the relationships of which, one with 

 another, Garstang was silent, Sphaeronectes, as the ancestral type, comes first. It is followed by the 

 Prayidae, which appear to be primitive in that the adults resemble the larvae fairly closely in their 

 organization. The Hippopodiidae develop heteromorphous definitive nectophores, but their larvae 

 are very similar to those of the Prayidae. These are followed in my list by Moser's group, the Tribus 

 Diphyidae Intermediae, which appears to lead on to the genus Diphyes (sensu stricto) and to the 

 Abylidae. The relationships inter se of the remaining Diphyidae are unknown, except that the species 

 of Galettinae seem to be related to one another. It still remains to be ascertained whether, as I suspect, 

 there is a fundamental difference between those species — included in the three groups Tribus 

 Diphyidae intermediae, Diphyes {sensu stricto) and Abylidae on the one hand, and the remaining 

 Diphyidae on the other, the suspected difference being, that in the former three groups, the larval 

 nectophore is retained as the anterior, definitive one, whilst in the last group it is caducous and is 

 followed by one or more heteromorphous ones. 



NUMBER OF SPECIES 

 In his famous ' Challenger ' Report Haeckel dealt with about sixteen species taken by ' Challenger ', 

 eleven of them very common, and two not seen since. In that report he described and figured altogether 

 about thirty distinct species of Siphonophora. Haeckel listed 240 so-called species, of which not more 

 than fifty are recognized as distinct to-day. Twenty-six of these 240 he called new species, which he 

 promised to describe later. Four of them prove to be synonyms for known species and the rest are 

 nomina nuda, since Haeckel never did describe them. 



No one, as far as I know, has commented on the fact that there are so few species — something in 

 the order of 150 — of Siphonophores, which occur in great abundance in most seas. Is the small 

 number of species due to the fact that so many of the species have very wide distributions and so 

 consist of virtually single, interbreeding populations? 1 There is a second remarkable phenomenon, 

 namely the existence of many genera with 'pairs' of species such as Abylopsis tetragona and 

 A. eschscholtzii; Diphyes dispar and D. bojani, Rosacea cymbiformis and R. plicata, Chelophyes 

 appendiculata and Ch. contorta, Agalma okenii and A. elegans, Eadoxoides tnitra and E. spiralis, 

 Chuniphyes multidentata and Ch. moserae, Nanomia bijuga and N. car a, Rhizophysa filiformis and 

 Rh. eysenhardtii. The number of monotypic genera is also large, Apolemia, Lychnagalma, Erenna, 

 Nectalia, Physophora, Athorybia, Melophysa, Dromalia, Rhodalia, Physalia, Porpema, Porpita, Velella, 

 Enneagonum and Bassia. These two categories account for thirty-five out of the total of 150 species, 

 but the analysis cannot be completed at present. There has appeared recently a note by C. B. Williams 

 (195 1) on the relative sizes of genera in the classification of animals and plants. ' One general principle ', 

 he said, ' of a mathematical nature, has several times been pointed out and discussed. In almost every 

 classification the number of genera with one species is greater than with two, the number with two 

 greater than with three, and so on ; so that if we plot the classification in the form of a frequency curve, 

 we get a " hollow curve " somewhat resembling a hyperbola.' He then proceeded to show that we can 

 only recognize as real the scheme of relative specific and generic relationships commonly used by good 

 systematists if we bear in mind that the point in time-past, at which to 'draw the line' or make 



1 If it can be shown that waste products play some part in causing mutation, it seems possible that there may be a connexion 

 between the small number of species and the fact that waste products must quickly diffuse out into the surrounding sea-water. 



