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



brown, sometimes more orange, at other times more violet or even black. The inferior 

 margin of the hemispherical or campanulate pigment-cap is usually separated by a 

 sharp circular boundary-line from the colourless or yellowish middle portion of the 

 air-sac (PL XXIV. fig. 5, pp). 



Sacculus pericystalis. — The greater middle part of the pneumatosac is a simple 

 layer of exodermal epithelium, and produces the pneumatocyst as a true cuticle ; it 

 ends, together with this latter, on the pylorus infundibuli, or the opening by which the 

 air-flask communicates with the funnel-cavity; the terminal edge of the cuticle which 

 surrounds this circular pylorus is usually a thickened chitinous ring (annulus infun- 

 dibuli). 



Pneumatochone (infundibulum pneumatophori, " Lufttrichter," Chun, 48, p. 512). — 

 The distal or basal part of the original pneumatosac situated below the pylorus is the 

 important pneumatochone or the "hypocystic funnel." The thickened glandular 

 epithelium which lines its cavity is very different from that of the pericystic sac. It 

 produces no cuticle, but is composed of several layers of polyhedral exoderm-cells, 

 which have a rather dark granular protoplasm and a peculiar yellowish or more greenish 

 appearance (PL XXIV. fig. 7). From this ovate, hemispherical, or nearly spheroidal 

 " air-funnel " arises inside, the endocystic tapetum, and outside, the clustered groups of 

 hypocystic villi (PL XXII. fig. 7). 



Tapetum endocystale ("secondary exoderm," Chun, 48, pp. 514, 530). — The exo- 

 dermal tapetum which lines inside the basal half of the pneumatocyst (in young 

 Rhizophysidse the basal third, in adult more than two-thirds) is the direct continuation 

 of the hypocystic funnel, and its function is, Like that of the latter, the secretion of gas ; 

 both together represent the " pneumadenia," or the gas-secreting gland. The stratified 

 exoderm of the air-funnel grows upwards, passes through the chitinous ring of the 

 funnel-pylorus into the cavity of the pneumatocyst and lines the greater part of its 

 inside, with the exception of the apical part which is covered by the mitra ocellaris. The 

 endocystic tapetum is composed of several strata of the same peculiar granular and 

 yellowish or greenish exoderm-cells, as those which line the funnel-cavity and are 

 polyhedral by mutual compression (PL XXII. fig. 7,pd; PL XXIV. fig. 7,pd). 



Villi hypocy stales (PL XXIV. figs. 4, 5, pv, 6). — The basal portion of the 

 pericystic cavity, and often its greater part (with the exception of the apical portion), is 

 filled with bunches of clustered villi, which arise from the outside of the hypocystic 

 funnel. They were first described by Gegeobaur (in 1854) as " csecal diverticula " of the 

 pneumatocyst (" blinddarmiihnliche Fortsatze," 7, p. 326, Taf. xviii. fig. 6, e). Huxley 

 (in 1859) described them as "elongated and more or less branched processes, which 

 project from the distal surface of the pneumatocyst freely into the cavity of the 

 pneumatophore ; each process consists of a cellular axis, invested by the ciliated 

 entoderm. The cells of the axis are clear and very large, and have an opaque oval 



