REPORT ON THE SIPHONOPHOR.^. 15 



As in the latter, each siphon or " suctorial tube " possesses at the distal end of the sac- 

 shaped body a mouth opening, at the proximal end a stem opening, through which the 

 simple cavity communicates with the cavity of the umbrella or the stem. By one or two 

 (often even three) annular strictures the simple cavity of the siphon is divided into 

 two, three, or four chambers. When four portions can be distinctly distinguished, 

 the first (proximal) is a thin stalk (pedunculus siphonalis), usually bearing the base 

 of insertion of the tentacle ; the second is a thick-walled crop (" Vormagen," basigaster) 

 equipped with masses of urticating cells ; the third is the stomach proper (stomachns) , 

 usually with "hepatic stripes," more rarely with glandular villi; the fourth is an ex- 

 tremely contractile proboscis. The stomach usually passes without sharp boundary into 

 the proboscis, but is generally definitely marked off from the basigaster, often by means 

 of a pyloric valve. The stalk of the crop region is often degenerate or not distinctly 

 defined. The single tentacle springs from this portion in the majority of Siphonanthse. 

 According as the adult Siphonophoral corm has only one or several siphons, we distinguish 

 Monosiphonise and Polysiphonise. 



MONOSIPHONLE or MONOGASTRIC SIPHONOPHOR/E. 



Of great importance for the right understanding of the Siphonophorse is the fact, 

 that in all forms the primary medusiform larva (Disconula or Siphonula) bears only a 

 single siphon, and that this remains in one portion of the class, namely in the 

 Monogastricae, while in the other division it is modified into the stem, and is physio- 

 logically replaced by numerous secondary siphons (lateral branches of the latter). 

 Hitherto the monogastric Siphonophorse have been represented solely by the Eudoxise 

 among the Calycophoridse ; but among the new deep-sea Siphonophorse of the Challenger 

 Expedition which I have described in this Report, there are interesting monogastric 

 forms from three other orders (Discahdse, Athoridae, Cystalidae). Since in both legions of 

 the class the Monosiphonise exhibit close resemblance to the known larval forms of the 

 Polysiphonise, they may be regarded as " sexually mature larvse." In more accordance, 

 however, with the phylogeny is the reverse interpretation, that those larvse of the 

 Polysiphonise repeat, according to the fundamental biogenetic law, the structure of 

 their Monosiphonial ancestors. 



In regard to the position of the stomach on the subumbrella, the Monosiphonise 

 exhibit in both legions very important differences, which are explained by their diphyletic 

 origin. In the Discalida? or monogastric Disconanths, the primary siphon occupies 

 the central point of a regular octoradial umbrella ; in the monogastric Siphonanths, 

 on the other hand, it is more or less excentrically situated at the base of a bilateral 

 umbrella. 



