110 



COMPARATIVE ANATOMY 



CHAP. 



these three parts umbrella, tentacle, and gastric peduncle. This 

 medusoid larva is bilaterally symmetrical. Its umbrella has a deep 

 cleft, it possesses only one tentacle, its gastric peduncle is filled with 

 yolk. The Siphonophora colony arises on the gastric peduncle by 

 gemmation. 



Let us now consider the polymorphic appendages of the Siphonanth 

 stem, which we compared with the daughter Medusce budding on the 

 gastric peduncle of Sarsia siphonophora. All these appendages are 

 arranged on the stem in a line whose position is called ventral. The 





FIG. 84. Stephalia corona, after Haeckel. A, Halved longitudinally. B, From life, sb, Swim- 

 bladder ; au, aurophore ; sg, swimming-bells ; fca, canal system of the stem (chief stomach, st) ; go, 

 gonophore clusters ; o, aperture (mouth) of the peduncle (chief gastric tube, st) ; hy, gastric tubes 

 (siphons) ; /, tentacles. 



line generally becomes a spiral, because of the spiral twisting of the 

 stem. Highest up on the stem under the pneumatophore (when one 

 is present) the so-called swimming-bells or neetophores are inserted ; 

 these are wanting only in the Cystonecta. The swimming-bells exclu- 

 sively and alone provide for the locomotion of the whole stock. They 

 have lost all those Medusa organs which were of no use to or even 

 hindered the fulfilment of this function, first of all therefore, mouth, 

 gastric peduncle, and tentacles. The locomotory organ of the Medusa, 

 the disc or umbrella, however, is all the more strongly developed ; it 

 is much vaulted with a strong circular muscular layer in the sub- 

 umbrella. Its edge projects in the shape of a true velum. At the 

 base of the velum runs the circumferential canal, into which 4 radial 

 canals enter. The swimming-bells are so inserted on the stem by their 



