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



spheric air, taken in by the stigmata of the exumbrella ; (3) this air is conducted through 

 the respiratory tracheae to the different appendices of the suburnbrella (central siphon 

 and gonostyles) ; (4) the distal or lower ends of the tracheae are closed. 



The observations which I have myself been able to make on the structure and 

 function of the pneumatophore in the different groups of Disconectae have conducted 

 me to quite opposite views, viz., — (1) The Disconectae are (all or partly) capable of 

 sinking under water, by muscular compression of the pneumatocyst, and expulsion of 

 air by the stigmata of the exumbrella. (2) The air contained in the pneumatocyst is 

 not atmospheric air taken in by the stigmata, but a gas produced by the exodermal cells 

 of the great "central pneumadenia" (the so-called "liver" or central organ). (3) This 

 gas, therefore, has not respiratory, but hydrostatic functions (like the gas in the 

 swimming-bladder of the fishes). (4) The distal or lower ends of the tracheae are not 

 closed by a chitinous plate, but open into the glandular, gas-secreting, exodermal 

 tissue of the centradenia ; they are comparable to the " pylorus infundibuli " of the 

 Siphonanthae. 



Central Siphon. — The large central polypite, which is placed in the centre of the 

 suburnbrella in all Disconectae, is the original manubrium of the primary medusiform 

 larva (Disconula) ; its terminal mouth is the permanent primary mouth of the latter. 

 The central siphon is the only organ of feeding and digesting in the monogastric family 

 Discalidae, whilst in the other two families of the order, the polygastric Porpitidae and 

 Velellidae, this function is also executed by the numerous peripheral gonostyles, which 

 are here developed in the form of mouth-bearing siphons or secondary polypites. But 

 also in the young monogastric larvae of these latter, the primary central siphon is alone 

 provided with a mouth. Its size is very variable in the different Disconectae ; generally 

 it is comparatively large in the small Discalidae, and in the smaller forms of Porpitidae 

 and Velellidae, which possess few secondary siphons ; on the other hand, it is relatively 

 small in the larger forms of the two latter families, which possess a great number of 

 feeding peripheral polypites. 



The form of the central siphon in the Discalidae and Porpitidae is inversely conical, 

 with circular transverse section ; whereas in the Velellidae the inverted cone is stronelv 

 compressed from both sides, so as to be elliptical or lanceolate in transverse section. The 

 wider proximal or upper part, or the true stomach, passes without a sharp boundary into 

 the cylindrical, very contractile, distal or lower part, the proboscis. The latter, as well 

 as its distal opening, the mouth, is very variable in size and form, according to its state 

 of contraction. 



The surface of the central siphon exhibits in many Disconectae a number of longi- 

 tudinal or radial folds, visible partly outside, partly inside ; usually there are eight or 

 sixteen, sometimes more. The distal mouth correspondingly often exhibits eight or 

 sixteen lobes, sometimes also four larger lobes ; at other times it is simply circular, or 



