CNIDARIA ORGANISATION OF SIPHONOPHORA 



111 



aboral or apical poles, that the aperture of the bell is turned downwards 

 and outwards, away from the apex of the stem. 



When the swimming-bells contract, and so expel the water down- 

 wards out of their subumbrellar cavities, the whole stock is propelled 

 by the recoil in the opposite 

 direction, i.e. upwards. The swim- 

 ming-bells are not regularly, but 

 bilaterally symmetrical, which is 

 explicable by their insertion on 

 the stem, and by the position they 

 have to assume to effect the motion 

 forward of the whole stock. As 

 to the number and arrangement of 

 the swimming-bells, we find either 

 one or two opposite each other, 

 or several, or often very many, 

 arranged in two or many rows in 

 circles round the stem. The points 

 of insertion of the bells, however, 

 always lie in a spiral, which is 

 sometimes much extended, some- 

 times much compressed. The 

 direction in which this spiral is 

 twisted is the opposite of that 

 in which the other appendages 

 are arranged. The radial vessels 

 of each swimming - bell, which 

 unite at its apical pole, are in 

 open communication with the 

 endodermal axial canal of the 

 stem. 



Beneath the swimming-bells the stem carries the following different 

 kinds of appendages, which we may also consider as modified Medusae : 



A. The gonophores or reproductive persons (Fig. 83, go v go 2 , go 3 ). 

 To these belong exclusively the function of forming the sexual 

 products. They are either male or female. The typical organisation 

 of a Craspedote Medusa is still more or less faithfully maintained in 

 them. They possess a bell-shaped umbrella with velum, circumferen- 

 tial canal and radial canals, and, further, a gastric peduncle which 

 projects into the subumbrellar cavity (occasionally with an oral 

 aperture in addition), and in whose wall, as in the Codonidce among the 

 Craspedote Medusce, the sexual products arise. The umbrella is here, 

 probably, a protective apparatus. Occasionally the rudiments of 

 tentacles are still found on the margin of the disc. Sometimes, how- 

 ever, the whole Medusa form is considerably degenerated. 



B. Sterile persons. These perform the functions of taking in food, 

 and digestion, of protection, touch, etc. The Medusa structure in them 



FIG. 85. Praya galea, after tfaeckel. 



