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



In order to get a clear conception of the bilateral promorph of the Siphonanthge 

 (very unnaturally described by many authors), it is very important to distinguish the 

 three primary dimensive axes, and to compare them with those of man or of some other 

 bilaterally symmetrical animal. The first or principal is the vertical main axis, the 

 longitudinal axis of the tubular stem ; its upper or apical pole bears the pneumatophore 

 in the majority of Siphonanthas, the primary nectophore in the Calyconectae. The 

 opposite end is the lower or basal pole. 



The second dimensive axis is the sagittal diameter ; its ventral or anterior pole is 

 marked by the series of buds, and usually by a ventral groove of the tubular stem. The 

 opposite dorsal pole is distinguished in the Auronectae by the aurophore, in Physalia by 

 the crest of the pneumatocyst. The vertical plane, which is determined by the sagittal 

 and the principal axes, perpendicular one to another, is the median or sagittal plane ; it 

 separates the right and left halves of the body. 



The distinction between right and left halves (often confounded by different authors, 

 and described in striking contradiction) is always clear, when the ventral side is 

 constantly defined in the same sense as that side of the body from which the buds arise. 

 Therefore, the two poles of the frontal diameter, or the third dimensive axis (right 

 and left pole), must be always the same. In the monogastric Calyconectae, for. instance 

 [Eudoxia, &c), the single siphon is placed on the ventral side of the bracj (or the 

 modified umbrella) ; in the polygastric Calyconectse, correspondingly, the trunk is placed 

 on the ventral side of the first or proximal nectophore (the nectosarc, therefore, on its 

 dorsal side). In the Diphyidse, the ventral sides of the two nectophores are opposed one 

 to another. 



The bilateral promorph of the Siphonanthse is at the same time quadriradial (or by 

 duplication of the parameres octoradial). This radial structure, inherited from the 

 ancestral quadriradial Medusas, is not only evident in the four radial canals of the 

 gonophores and nectophores, the eight hepatic stripes and mouth-lobes of many siphons, 

 &c, but also in the structure of the primary larval umbrella, and the pneumatophore 

 arising from it. The majority of the Siphonanthae exhibit in the basal part of the 

 pneumatosaccus eight (more rarely four or sixteen) radial pouches, which are separated 

 by vertical septa and comparable to the radial canals of a Medusa. 



