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



there arises early a vertical longitudinal fold of the exoderm above the pneumatocyst ; 

 and this commencement of the typical vertical sail determines the amphithect or 

 bilateral development of this family ; the umbrella becomes elliptical, and the gonostyles 

 bud between central siphon and corona of tentacles, not in concentric circles (as in the 

 Porpitidae) but in elliptical or oblongish rings. The ontogenetic metamorphosis of these 

 larvae, developing into the first asexual generation, is not completely known in any 

 Diseonectae. For further particulars regarding the ontogeny reference may be made to 

 the works of Kolliker (4), Vogt (5), Gegenbaur (7), Huxley (9), Pagenstecher (55), Alex- 

 ander Agassiz (57), and others. 



Phytogeny. — The historical or phylogenetic development of the various forms of 

 Sipkonophorae, which we unite in the legion Diseonectae, may be partly recognised from 

 the critical study of their comparative anatomy and ontogeny. By this means I have 

 been conducted to new views of the origin of this interesting subclass, quite different 

 from those of all naturalists who have hitherto treated the question. The general 

 opinion is that the Diseonectae are the most highly developed Siphonophorse, terminating 

 the series of this class, and most nearly related to the Cystonectae (Physalidae). Even 

 the radial chambers of the pneumatophore of the Diseonectae have been often compared 

 to the crista-chambers of the pneumatophore of Physalia. This comparison, and all the 

 important consequences deduced from it, are, in my opinion, perfectly erroneous. In 

 direct contradiction to it, I am convinced of the truth of the new theory which I have 

 already shortly explained in my propositions ; ! its principal points are here repeated. 



The Diseonectae (or Disconanthae) have no direct relation to the Siphonanthae (or all 

 other Siphonophorae) ; they have originated, independently of the latter, in a different 

 way and from a different group of Hydromedusae. Whilst the Siphonanthae are probably 

 the offspring of the Anthomedusae (Codonidae), and their cormus developed by budding 

 from the ventral line of the original siphon, the Disconanthae, on the other hand, are 

 probably descendants of the Trachomedusae (Trachynemidae), and developed by budding 

 from the subumbrella. 



The common ancestral group of all Disconectae is the family Discalidae (most nearly 

 allied to the Trachynemidae). From these, probably, the two other families, Porpitidae 

 and Velellidae, have been developed as two divergent branches, or, perhaps, the latter 

 have been derived directly from simpler forms of the former family. 



Synopsis of the Three Families of Diseonectae. 



1. Umbrella circular and regular octoradial. Blastostyles without mouth, . . 1. Discalidae. 



2. Umbrella circular, in the centre octoradial. Blastostyles with a mouth, . . 2. Porpitidae. 



3. Umbrella elliptical or bilateral. Blastostyles with a mouth, . . .3. Velellidas. 



1 System der Siphonophoren, Jena, 1888. 



