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



Somatocyst (figs. 5-8, cs). — The somatocyst is spherical, very large, and occupies the 

 apical half of the ventral part of the first nectophore. It is filled with large vacuolate 

 entoderm cells and therefore appears reticulate. It is connected at its dorsal side by a 

 small inferior descending canal with the top of the stem (aa), and by a small superior 

 ascending canal with the oleocyst (co). The somatocyst is twice as broad and half as 

 long as the nectosac (w). The oleocyst (co) is a small cascal process filled by an oil- 

 globule, and is nearly in contact with the middle of the coryphal crest (nk). 



Basal Necto})lwre (fig. 1, ventral view ; fig. 2, dorsal view ; fig. 3, lateral view from 

 the right hand ; fig. 4, from the left hand side). — The second nectophore (also called the 

 distal, posterior, inferior or basal nectocalyx), is four times as long and twice as broad as 

 the first, and of a very different form. It is in general a slender, pentagonal, truncate 

 pyramid, the five edges of which are asymmetrically developed and somewhat spirally 

 twisted. The ground-form is internally (with respect to the four radial vessels of the 

 nectosac) quadriradial ; externally (with respect to the five edges of the exumbrella) 

 quinqueradial ; and at the same time bilateral, with respect to the sagittal plane, which 

 divides the body into right and left halves, and is determined by the ventral position 

 of the hydrcecial canal, and the dorsal position of the nectosac (w). 



The five edges of the truncate pyramid are more or less wing-like, prominent, and 

 finish at the distal end in five triangular pyramidal teeth ; they are elegantly denticulate 

 in the lower third, and twisted in a cleltoidal (or right-hand ascending) spiral. Their 

 development is unequal and asymmetrical. The odd dorsal edge (nd), which runs along 

 the. dorsal median line of the nectosac, is less prominent than the four others, which are 

 paired. The broadest wing, and the strongest terminal tooth, are developed from the 

 left ventral edge (n 3 ), and next to this from the right dorso-lateral edge (n 2 ) ; whilst 

 the right ventral edge (n*), and the left dorso-lateral edge (n 1 ) are smaller. 



The two dorso-lateral edges (n 1 , n 2 ) correspond to the two symmetrical lateral 

 edges of a bilateral and quadriradial Medusa, whilst the two ventro-lateral edges are 

 produced by the development of the hydrcecial canal on the ventral side of the second 

 nectophore. This canal is not closed, but covered by the two broad wings which develop 

 from the two ventro-lateral edges ; the right of these (nx) is broader and overlaps the 

 left (nl) ; their inferior free margin is dentate. 



The truncate apical or proximal face of the second nectophore is obliquely bevelled, 

 and bears a triangular apophysis, which enters into the hydrcecium of the first necto- 

 phore, and encloses the nectocalycine duct. The five edges are more equally developed 

 at this superior end (fig. 10) ; a horizontal transverse section beyond it (rig. 11) 

 demonstrates how the two ventro-lateral wings develop more strongly, and form 

 the hydrcecial canal (ui). The differentiation of the five wings is strongest at the distal 

 or inferior end. 



The basal or distal face of the second nectophore (seen from below in fig. 12) exhibits 



