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



NECTOSOME and SIPHOSOME. 



In all Siphonophorse, without exception, both in the monogastric and polygastric 

 forms, the developed corm may be distinguished, both from a morphological and from a 

 physiological point of view, into two main portions, the nectosome and siphosome. The 

 nectosome or the swimming body forms in the Siphonophorse which swim quietly on the 

 surface of the sea, the upper, anterior or proximal portion of the stock ; the siphosome 

 or nutritive body the lower, posterior or distal portion. Physiologically the former is 

 the organ of locomotion (and often also of respiration), the latter, on the other band, the 

 organ of nutrition and reproduction. If one compares the Siphonophoral stock with the 

 simple medusoid person, then the nectosome in general represents the umbrella, and the 

 sipfiosome of the corm the manubrium of the Medusa. In the polygastric Siphonanths 

 the two portions pass through an independent course of development, separated from one 

 another, with distinct points of growth. In those Siphonanths where the stem is spirally 

 coiled, the spiral twisting is usually opposite in the two portions ; the nectosome has 

 usually a left-handed ("Lambda"), the siphosome a right-handed ("Delta"), spiral. 



NECTOSOMES of the FIVE ORDERS. 



The swimming apparatus of the Siphonophorse is formed of two essentially different 

 organs, the swimming-bell (nectophore) and the swimming-bladder (pneumatophore). The 

 nectophore is the umbrella of a Hydro-medusa, in which both the annular muscular layer 

 of the velum and of the subumbrella, and the primary canal-system (four radial canals 

 united by a marginal circular canal) are perfectly developed. The pneumatophore, on the 

 other hand, is an invaginatcd and much modified umbrella, in the surface of which (by 

 apical or lateral invagination of a gas-gland) a chitinous gas-filled "air-flask" or pneu- 

 matocyst is formed. The definition of the five Siphonophoral orders is in the first place 

 determined by the different formation of the swimming apparatus. 



I. The CalyconectaB or Calycophoridse possess only one, two, or more swimming-bells, 

 but no pneumatophore. 



II. The Cystonectse or Pneumatophoridse bear only a simple large pneumatophore, 

 but no swimming-bells. 



III. The Disconectae or Ohondrophoridse jjossess an octoradial pneumatophore, usually 

 composed of concentric annular chambers, but no swimming-bells. 



IV. The Physonectse or Physophoridse bear on the apex of the stem a simple 

 pneumatophore, and under that a biserial or multiserial column of swimming-bells ; 

 sometimes instead of these a corona of numerous bracts. 



V. The Auronectse or Aurophoridse (a new and hitherto quite unknown group of 

 very remarkable deep-sea forms) possess on the apex of the stem a colossal pneumato- 



