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



row has five or six developed nectophores, besides some young buds at the apical 

 blastocrene, below the pneurnatophore. Their total number, therefore, is twenty to 

 twenty- four. Their size increases gradually from the top towards the base of the necto- 

 some ; the lowermost are the largest. The transverse section of the nectosome, or the 

 apical view (fig. 15), exhibits a cross composed of four nectophores, lying in different 

 horizontal planes according to the spiral twisting of their line of insertion. This lseotropic 

 spiral line is the ventral median line of the trunk. When the nectophores are detached 

 from the trunk (figs. 9-13), this latter (an) appears as a spindle-shaped bladder about 

 four times as long as thick, and tapering towards the two constricted ends ; in its ventral 

 median line arises a thin membranous vertical lamella, the mesentery of the nectosome, 

 which bears the contracted pedicles of the detached nectophores (np). 



Pneumatophore (figs. 1, 9-13, p).- — The apical float is ovate, 10 mm. long and 5 mm. 

 broad, with a pink octoradial pigment-star at the top. Eight equidistant meridian lines 

 connect the two poles of its vertical axis, and are visible outside, as very delicate threads 

 in the upper, and broad bands in the lower half. These lines are the insertions of eight 

 meridian ribs on the inside of the pneumatocodon, which connect it in the lower half 

 with the pneumatosaccus, and so form eight radial pouches. The breadth of the vertical 

 radial septa separating the latter is effected by eight apophyses of the exodermal 

 pneumadenia, extending in the fulcrum of the septa, which are covered by entoderm. 



Nectophores (figs. 1 and 4, basal view ; fig. 2, lateral view, from the right side ; 

 fig. 3, dorsal view). — The bilaterally symmetrical umbrella of the largest nectophores is 

 twice as long and broad as thick ; its principal axis (directed obliquely from above and 

 inside downwards and outwards) is 12 mm. in length, and the frontal axis the same, 

 whilst the sagittal axis is only 6 mm. In the median line of the concave ventral side 

 arises the lamellar pedicle (fig. 2, np), the superior apex of which attaches the nectophore 

 to the ventral mesentery of the trunk. The ventral edge of this pedicle fits into a 

 corresponding median groove in the dorsal side of the subjacent nectophore. Seen from 

 the base (figs. 1, 4), the outline of the nectophore is nearly rectangular, with a deep 

 furrow in each side. The proximal half of the nectophore is much broader than the 

 distal half, and dilated in the form of two ovate lateral lobes or auricles (fig. 3). To 

 these correspond two shorter triangular basal lobes, which arise from the ventral side of 

 the ostium of the umbrella. 



Nectosac (figs. 1-4, w).- — The muscular subumbrella of the nectophores is relatively 

 large, since their jelly umbrella is rather thin-walled ; the form of the latter is determined 

 originally by the development of the former. The nectosac is composed of three different 

 parts, which have an irregular ovate form ; an odd median part and two paired lateral 

 lobes in the proximal half. The nectocalycine duct, which passes through the pedicle 

 of the umbrella, enters into the ventral side of the nectosac below its top. It divides 

 into four radial canals of very different shape, two odd sagittal and two paired lateral. 



