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



ing in all these Cnidariae. The Scyphomedusae or Acraspedae, as well as the class of Corals 

 or Anthozoa are descended from the Scyphopolyps (Scyphostoma, Spongicola, &c) ; both 

 these classes have gastral filaments or mesenteric filaments, which have arisen from the 

 taeniola of the Scyphopolyps. 



§ 6. Polyphyletic origin of the Medusae. That the class of Medusas belongs to the 

 polyphyletic classes of animals is now a phylogenetic hypothesis, which may be brought 

 forward as a probability bordering upon certainty, although, on the one hand, the character- 

 istic structure of the Medusae appears so uniquely organised that they are most suitably 

 placed as a separate class in the system of the animal kingdom, yet, on the other hand, it 

 by no means follows that they are all derived from a single common ancestral form, which 

 already possessed the form of the Medusae. It is much more probable that the two sub- 

 classes, or sections of this class, the Craspedotae and Acraspedae, are of separate origin, and 

 are descended from groups of Polyps, which have developed into Medusae, independently 

 of one another. A strong support to this hypothesis is, that the Scyphopolyps, the ancestral 

 form of the Acraspedae (Scyphostoma, Stephanoscyphus, &c), already possess the four im- 

 portant interradial taeniola or gastral longitudinal ridges from which the four character- 

 istic groups of filaments are developed in all Acraspedae (or Scyphomedusae). On the 

 other hand, the characteristic groups of filaments are wanting in the Craspedotae (or 

 Hydromedusae) as the typical four interradial taeniola are wanting in their ancestral 

 form, the Hydropolyps. Moreover, the reproductive organs originate in the Craspedotae 

 (as in most Hydropolyps) from the ectoderm and in the Acraspedae (as in the Scyphopolyps 

 and Corals) from the endoderm. As regards the two sections or sub-classes, the Craspedotae 

 are more probably of monophyletic origin, the Acraspedae of polyphyletic. 



§ 7. Orders of the Craspedotae (System, pp. 2, 233, 360). The section of the 

 Craspedotae or Hydromedusae is divided into two sub-sections and four orders. The two 

 sub-sections, Leptolinae and Trachylinae, are thoroughly and pre-eminently distinguished 

 from one another by the absence or presence of the cordyli or tentacular "auditory clubs." 

 These are modified acoustic tentacles, consisting of a solid axis of chordal endodermal cells, of 

 which the last (distal) contains one or more otolites ; their ectodermal epithelium bears stiff 

 auditory bristles. The first sub-section (or Acordyliae) has no auditory clubs ; it has, in 

 fact, either no auditory organs or only " velar auditory vesicles" (marginal vesicles on the 

 velum with ectodermal otolites) which are quite different from the cordyli, and occur in no 

 other group. Moreover in the Leptolinae, the tentacles are usually hollow, very movable 

 contractile filaments, and their velum is delicate and thin. The second sub-section, the 

 Trachylinae (or Cordyliotae), on the other hand, invariably bear true cordyli or auditory 

 clubs with endodermal otolites on the margin of the velum ; their tentacles are, moreover, 

 usually solid, tolerably stiff, and slightly contractile filaments, and their velum is thick and 

 compact. The Leptolinae mostly develop indirectly (by metagenesis), the Trachylinae 

 mostly directly (by hypogenesis). In both sub-sections there is an order with gastral geni- 



