336 THE VOYAGE OF H.M.S. CHALLENGES. 



after the emission of gas through the apical pore, it represented a depressed spheroid 

 of 3 to 4 mm. only. The pneumatocodon, or the outer wall of the pneumatophore 

 (fig. 7, pu), is separated from the inner wall, or the pneumatosaccus (fig. 7, ps), by a wide 

 cavity (p>c). This pericystic cavity opens below into the stem-cavity of the siphosome 

 (at), whflst it is closed above, surrounding like a ring the apical pore (p>o) where both 

 walls are connected. The distal or inferior half of the pericystic cavity is filled by the 

 numerous finger-shaped hypocystic villi (pv), arising in eight radial bunches from the air- 

 funnel (pi). 



Pneumatocyst (figs. 6-8, pf). — The chitinous air-flask is an ovate bladder, suspended 

 from the apex of the surrounding pneumatophore, and hanging down freely into its 

 cavity. Its cuticular wall has two opposite openings on the poles of its axis. The 

 superior or proximal opening is the apical stigma (po) serving for the emission of gas when 

 the animal wishes to sink down ; it may be closed by the sphincter stigmatis, a strong 

 ring-muscle, the antagonist of which is a corona of radial muscles, opening the stigma. 

 The inferior or distal opening of the air-flask is the pylorus infundibuli (py), by which 

 its cavity communicates with the subjacent air-funnel (pi). The convex outside of the 

 pneumatocyst is covered by the simple exodermal epithelium of the pneumatosac (ps), 

 and in the upper third by the mitra ocellaris, a purple hemispherical cap composed of 

 elegant polygonal pigment-cells (pp). The concave inside of the air-flask is naked in the 

 upper third (covered by the ocellar mitra, pp) ; it is lined in the two lower thirds by 

 the endocystic tapetum (pel), a stratified glandular epithelium composed of the same 

 yellowish-green exoderm cells as line the cavity of the subjacent spheroidal air-funnel 

 (infundibulum, pi). The greenish glandular epithelium is the important pneumadenia 

 and secretes the gas. 



Hypocystic Villi. — The air-funnel (infundibulum or pneumatochone, pi), which 

 forms the blind distal portion of the air-sac, is surrounded by a regular corona of eight 

 radial clusters, composed of numerous branched hypocystic villi. Each villus is com- 

 posed of a few colossal giant-cells of the exoderm (arising from the outside of the 

 pneumadenia) and covered by a vibratile epithelium of small ciliated entoderm-cells 

 (PI. XXIV. fig. 6). The diameter of the vesicular giant-cells is 1 to 2 mm., and their 

 nuclei, when stained by carmine, are visible to the naked eye. The further structure 

 and the physiological function of these parts are the same as in the Ehizophysidse, 

 (described above, pp. 310, 320). 



Siphons (fig. 6, s).— The large feeding polypites are 10 to 15 mm. long, very 

 movable spindle-shaped tubes with a thick muscular wall. The short basal pedicle 

 bears the tentacle on its dorsal side. The largest part is the dilated stomach, covered 

 inside with numerous yellow hepatic villi. The distal proboscis is very muscular ; its 

 mouth-opening may be expanded in the form of a circular suctorial disc, the margin of 

 which is divided into sixteen lobes. 



