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



narrow muscular air-duct (auroductus), which opens internally at the base of the 

 pneumatophore, and externally to the outside. The radial canals of the metamorphic 

 nectophore are modified into a complex radial system of wide glandular chambers. 



SIPHOSOME or NUTRITIVE BODY. 



The nutritive body exhibits in the different groups of the class Siphonophorae much 

 more manifold and important differences of form and structure than the swimming 

 body. Undoubtedly the most important difference is this, that in the Monosiphonise (or 

 the monogastric Siphonophorae) the archisiphon or protosiphon (the primary gastral tube 

 of the medusoid larva) remains alone as the organ for the reception and digestion of food, 

 while in the Polysiphonise (or the polygastric Siphonophorae), "secondary suctorial tubes" 

 or metasiphons are developed by budding, each provided with a gastric cavity 

 and a mouth opening. In the Polysiphonise the primary mouth opening of the medusoid 

 larva only rarely persists, in all Disconanthse, and in two families among the Siphon- 

 anthae (Stephalidse and Physalidae). In most (perhaps all the rest) of the polygastric 

 Siphonanthse, the primary mouth opening of the protosiphon is probably closed, and 

 the latter persists only as the stem of the stock (truncus or ccenosome). 



The polymorphic appendages, which bud out as lateral branches from the stem of 

 the siphosome, are partly medusoid (bracts, gonophores), partly polypoid (sip>hons, 

 palpons, cystous, gonostyles). The poly-organ theory regards the former as multiplied 

 umbrellas of one Medusa, or as secondary vicaria of such, the latter as repeated 

 manubria or vicaria of the same. The poly -person theory, on the other hand, regards 

 each single bract as a medusoid person, which has lost all its organs except the umbrella, 

 and each single siphon and palpon as an independent Hydropolyp. Our medusome 

 theory regards in the different cases these polymorphic appendages partly as dislocated 

 organs of medusomes, partly as multiplied reserve organs or vicaria of the same. 



SIPHONS or SUCTORIAL TUBES. 



{Polypites, Gastral Tubes, Stomach Sacs, Nutritive Polyps, Eating Polyps, 



Hydranth s, Gastrozooids. ) 



The siphons, which have given the name Siphonophorae to the entire class, are the 

 most important and the most constant appendages of their organism. From a physiological 

 point of view they are rightly regarded as organs for the reception of food and digestion ; 

 from a morphological point of view they are sometimes regarded as homologous 

 with an entire Polyp, sometimes with the gastric tube or manubrium of a Medusa. 



