REPORT ON THE SIPHONOPHOR^E. 283 



radially from the apical centre of the float and surround it like meridional arches (fig. 

 40). Sometimes eight stronger equidistant radial muscles are developed (fig. 33). The 

 fulcrum, or the supporting gelatinous plate, is a hyaline and structureless cartilaginous 

 lamella - 2 to 0"5 mm. thick; it is twice as thick in the peripheral as in the apical 

 part of the float. The fulcrum is pierced (mainly in the basal periphery) by a variable 

 number of simple or branched radial cords, which connect its inner and outer epithelium. 

 They are partly solid apophyses of the entoderm (fig. 30, d x ), partly nutritive canals ; 

 sometimes these canals seem to open on the outer surface of the pneumatocodon. 

 The stratum of ring-muscles which lies inside the fulcrum is thinner than the outer 

 layer of radial or longitudinal muscles ; but sometimes the circular muscles are also 

 well developed, and effect a horizontal annulation of the float. Stephonalia (PI. VI. figs. 

 32, 33), which exhibits outside eight strong radial muscle-bands, is distinguished also 

 by a peculiar development of parallel muscle-rings on the inside of the pneumatocodon. 

 .The entodermal epithelium which lines the inside is always composed of large cylinder- 

 cells (fig. 30, d). 



Pneumatosaccus. — The air-sac, or the inner (originally invaginated) wall of the pneu- 

 matophore, is of the same form as the somewhat larger surrounding pneumatocodon (or 

 the outer wall), separated from it by the pericystic cavity (fig. 24, ps), and connected with 

 it by the above-mentioned numerous trabecules. The inner wall is much thinner than the 

 outer, and composed of three plates only, viz. — (l) the entodermal epithelium composed of 

 high cylinder-cells (PI. V. fig. 24, di) ; (2) a rather thin, but firm and elastic fulcrum (the 

 structureless supporting plate, fig. 24, z L ) ; and (3) the exodermal epithelium composed of 

 much smaller cells (fig. 24, e^) ; this inner surrounds and produces the pneumatocyst. 



Pneumatocyst. — The air-flask of the Auronectse is a thin but firm and elastic cuticle, 

 secreted and surrounded by the pneumatosac, therefore of the same form and size. It 

 has no apical stigma, as in the Cystonectse, but is everywhere closed, except at the single 

 basal opening, which we call auropyle (PI. IV. fig. 16, li, seen from above ; PI. V. fig. 24, 

 li, seen in the sagittal section). This auropyle lies excentrically in a small circular 

 dimple on the dorsal base of the pneumatocyst, in its sagittal axis. The periphery of 

 this dimple (foveola auropylas) marks the internal boundary between the pneumatophoie 

 and the attached aurophore. The chitinous plate of the pneumatocyst seems to be 

 continued directly into the vagina pistilli (If). Compare below. 



Aurophore (PI. IV. figs. 15, 16, I; PI. V. figs. 24-28 ; PI. VII. figs. 39, 40, 48, I).— 

 The aurophore or air-bell of the Auronectse is a peculiar and most remarkable organ, 

 wanting in all other known Siphonophorse ; it seems to be the modified umbrella of a 

 medusome, or a peculiar medusoid person, which was originally a modified nectophore, 

 and adapted to the production and emission of the gas contained in the large pneumato- 

 phore. The form of the aurophore (/, PL I. fig. 1 ; PI. III. figs. 13, 14 ; PI. IV figs. 15, 

 16) is roundish, nearly globular or somewhat pear-shaped ; it is attached by a broad 



