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



which is preserved in the bilateral larva, Siphonula ; but partly the bilateral type is 

 produced by adaptation, and mainly by accommodation to the conditions of development 

 of the single forms. In order to get a clear conception of these difficult promorphological 

 relations, we must distinguish, firstly, the fundamental forms of the entire cormus and of 

 the single persons or medusomes composing it ; and secondly, the different promorpho- 

 logical development in the two independent legions of the class, the Disconanthae and 

 the Sqmonanthae. 



Tbe promorph of the single medusome has the quadriradial medusoid type usually 

 clearly expressed, when the umbrella is well preserved, as in the gonophores and necto- 

 phores (always with four radial canals and a connecting marginal ring-canal). But also 

 in other parts of the medusomes the radial promorph may be recognised, as in those 

 siphons which possess four, eight, or sixteen hepatic stripes, mouth lobes, &c. 



Promorph of the Conns in the Disconanthas. — The ideal geometrical fundamental 

 form exhibits in the colonies, of Disconanthae two different types ; one of these, 

 represented by the Discalidse and Porpitidae, is the primary and original type; the other, 

 exhibited by the Velellidse, is a secondary modification. All corms of Discalidae (Pis. 

 XLIX., L.) and of Porpitidae (Pis. XLV.-XLVIII.) preserve a completely regular 

 octoradial structure ; their ideal promorph is a regular octagonal pyramid. The vertical 

 main axis of this pyramid, around which the eight equal parameres are regularly 

 arranged, bears at its superior or apical pole the apical stigma of the central chamber of 

 the pneumatocyst, at its inferior or basal pole the mouth of the central siphon. The eight 

 equal sides of the pyramid are represented by the eight triangular radial chambers of the 

 pneumatocyst, whilst tbe eight perradial grooves between these, and the eight canals 

 running in the grooves, further the eight primary tentacles at the distal end of the canals, 

 mark the eight edges of the pyramid. The horizontal lines which connect these edges 

 with the vertical main axis are opposed in four pairs, and represent the four primary or 

 perradial cross-axes of the octagonal pyramid ; whilst the four secondary transverse axes 

 alternating with these, and bisecting the eight triangular radial girdle-chambers of the 

 pneumatocyst, are interradial. The perfectly regular octoradial promorph, which is so 

 clearly marked by the structure of the central pneumatocyst, is likewise expressed by the 

 entire structure of the canal-system, the eight primary perradial canals of the exumbrella, 

 the centradenia, the subumbrella, &c, by the regular octoradial corona of the eight 

 primary tentacles and gonostyles, the eight gastral ostia in the fundus of the central 

 siphon, the eight lappets of its mouth, &c. The young larva of all Discalidae and 

 Porpitidae (Disconula, PI. L. figs. 9, 10), and the simplest permanent genus of this legion 

 (Discalia, PI. XLIX.), exhibit tbe octoradial type in the same complete regularity as any 

 octoradial Medusa (e.g., Trachyncma, Pecti/llis). 



The Velellidse (Pis. XLIIL, XLIV.) differ from the regularly octoradial Porpitidae 

 and Discalidae in the amphithoct modification of the promorph, which is usually called 



