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



nectosome, in the superior or apical half of the corm, is composed of three different 

 portions: (l) a gigantic spheroidal or lenticular pneumatophore ; (2) a horizontal corona 

 of numerous radially arranged nectophores beyond the float ; and (3) a pyriform or 

 subspherical aurophore placed in the dorsal median line of the corona, probably an 

 enlarged pneumadenia. The siphosome, which occupies the inferior or basal half of 

 the corm, is composed of a large bulbous axial trunk and of numerous peripheral 

 cormidia, which cover its whole surface. Each cormidium bears upon a short common 

 pedicle a single siphon with a tentacle and one or two monoclinic gonodendra, provided 

 with one or more sexual palpons. 



Pneumatophore (p, PL I. fig. 1 ; PI. III. figs. 13, 14 ; PI. IV. fig. 15 ; PI. VI. figs. 

 32, 33 ; PL VII. figs. 39, 40, 48, 50). — The apical float filled with air is very volumin- 

 ous, comparatively larger than in any other known Siphonophorse, with the sole excep- 

 tion of the Physalidse. It is about half as large as the bulbous siphosome and has the 

 form of an inflated lens or a flattened spheroid, its vertical diameter (or central axis) 

 being usually only half as long as the horizontal diameter (or the transverse axis) ; the 

 former in the largest specimens of Rhodalia measures 10 to 11 mm., the latter 20 to 

 22 mm. The horizontal section of the pneumatophore (figs. 1, 16, p) is circular, the 

 vertical section (figs. 15, 40, 50, p) elliptical. The greatest part of its surface is free and 

 smooth. Only the lower surface is attached to the truncated proximal face of the stem 

 and separated from it by the flat horizontal hypocystic cavity (figs. 15, act, 40, ah). 

 The dorsal side of the pneumatophore bears at its base the large aurophore (I), whilst on 

 the opposed ventral side the set of buds is placed (PL IV. fig. 15, i). 



Although the development of the pneumatophore in the Auronectse is unknown, 

 there can be no doubt that it originates in the same way as in the Physonectse and 

 Cystonectse ; it represents, as in these latter, the modified umbrella of a Medusa, the 

 manubrium of which is the trunk of the siphosome (PL VII. figs. 40, 50). Since the large 

 air-sac is produced by an invagination of the apical part of the tubular trunk or 

 ccenosome, its wall is hollow, and the cavity of the wall filled by nutritive fluid ; the 

 pericystic cavity (PL V. fig. 24, ps) communicates below with the flat hypocystic cavity 

 (PL IV. fig. 15, aa). This latter is a simple circular or lenticular cavity without septa. 

 But the pericystic cavity is traversed by a variable number of irregular trabecule or 

 radial septa, which connect the thicker outer wall (pneumatocodon) with the thinner 

 inner wall (pneumatosaccus). 



Pneumatocodon. — The outer wall of the float (PL V. figs. 24, 30, z) is rather thick, 

 very firm and elastic, and is composed of five strata, viz. — (1) the outer exodermal 

 epithelium (e); (2) a subjacent layer of longitudinal or radial muscles ; (3) a thick fulcral 

 plate (2) ; (4) a thin stratum of circular muscles ; and (5) an inner exodermal epithelium 

 (d). The exodermal cell-layer, or the epidermis, is a thin and flat pavement epithelium. 

 The strong subjacent muscle-plate is composed of longitudinal fibres which diverge- 



