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



the Disconectse, on the other hand, have a complicated apparatus, composed of concentric 

 annular chambers, which occupy the greater portion of the' umbrella. In all cases 

 the pneumatophore arises very early in the primary medusoid larva, and that by a gland- 

 like invagination of the exoderm of the exumbrella. In the bilateral Siphonula of the 

 Siphonanths this invagination has an excentric position (being often shifted down near 

 to the umbrellar margin), but in the octoradial Disconula of the Disconanths it is 

 central, in the very apical pole. The marked and apparently considerable differences 

 of the developing pneumatophore, in relations both of time and space, between different 

 (often nearly related) Physonectse, I simply explain as secondary cenogenetic modifica- 

 tions. That portion of the medusoid larval body on which the first trace of it appears 

 always belongs originally to the exumbrella. 



In the Siphonanthse the invaginated portion of the exumbrella, comparable to a 

 simple, pouch-like, glandular sac, is known as the air-sac (jmeumatosaccus); it secretes in 

 its superior or apical half a chitinous membrane, the air-flask {jmeumatocystis) , while its 

 inferior or distal portion (the "air-funnel") discharges the function of a gas-gland 

 (pneumadenia). The glandular (usually yellowish or greenish) epithelium of this last 

 portion secretes the air, which passes by the basal opening of the air-flask (" funnel aper- 

 ture," " Trichterpforte," or pneumatopyle) into the latter. The Cystonectse or Pneumato- 

 phoridse possess at the apical pole of the pneumatocyst an external air-hole or stigma (the 

 primary opening of invagination), by which the air may be discharged at will. In many 

 Siphonanths the air-secreting glandular epithelium of the air-funnel (or infundibulum) 

 grows through the aperture into the basal half of the pneumatocyst and clothes the 

 latter as endocystal tapetum, or " secondary ectoderm" (Chun). 



In most Siphonanthse the air-sac becomes subsequently united with the peripheral 

 (uninvaginated) portion of the primary umbrella — the pneumatocodon — by a number of 

 vertical radial septa, usually eight, more rarely four or sixteen, and occasionally a variable 

 number. The radial pockets between these septa open inferiorly into the central canal 

 of the stem and represent the radial canals of a simple medusoid umbrella. On this is 

 based the opinion that the entire air-chamber is to be regarded as an "invaginated 

 swimming-bell" (Metschnikoff 1874). In contrast to this the air-chamber is at present 

 regarded by most zoologists as an independent medusoid person (or "medusiform zooid"), 

 and is supposed to originate as a "bud" from the primary larva (Leuckart 1875, Clans 

 1878, Chun 1887). The antithesis between these two opinions culminates in the inter- 

 pretation of the air-sac, which according to the first theory is the exumbrella of a medusoid 

 disc, according to the second the subumbrella. The latter opinion is according to my 

 conviction entirely erroneous, the former is in a certain sense admissible. The compara- 

 tive ontogeny of the Siphonophorse appears to me to show conclusively that the air-sac 

 is an apical gas-gland of the exoderm, which in the Disconula of the Disconanthas sinks 

 centrally into the gelatinous disc from the apex of the same inwards, and in the Siphonula 



