244 



CCELEN1ERATA. 



JTtJ 



D 



nophora physiologically as an organism and its appendages as 

 organs. In this connection we may mention that the sexual iriedu- 

 soid generation is so little independent that it only exceptionally 

 (Velellidce) reaches the morphological grade of the free-swimming 

 Medusa. 



In place of the attached and ramified hydroid-stocks we find in 



the Siphonophora a free-swimming con- 

 tractile unbranched stem (hydrosoma), 

 which is rarely provided with simple lateral 

 branches. The upper end of the hydro- 

 soma is frequently dilated to the form of 

 a flask (pneumatophore), and contains an 

 air chamber [pneumatocyst] (fig. 185). 

 In every case there is a central space in 

 the axis of the stem in which the nutritive 

 fluids are kept in constant motion by the 

 contractility of the walls and by the move- 

 ments of the cilia. The air sac or pneu- 

 matocyst at the apex of the hydrosoma is 

 connected to the chamber which contains it 

 by radial septa, and in many cases attains 

 a considerable size (Physalia). It func- 

 tions as a hydrostatic apparatus, and in 

 those forms, which have a long spiral 

 hydrosoma (Physophoridce), serves to keep 

 the body in an upright position. In some 

 cases the gaseous contents can escape freely 

 by one or more openings. 



The appendages which are attached to 

 the spirally twisted bilaterally symmetrical 

 stem, and whose cavities communicate with 

 that of the stem are of at least two kinds 

 (1) The polypoid nutritive animals with 

 their tentacles ;' (2) the medusoid sexual buds. The nutritive Polyps 

 (hydranths) are simple tubes provided with a mouth, and never 



Gegenbaur, " Neue Beitrage zur Kenntuiss der Siphonophoren," Nova Acta., 

 Tom. XXVIL, 1859. E. Leuckart, ' Zoologisclie Untersuchungen," I., Giessen, 

 1853. K. Leuckart, " Zur naheren Kenntniss der ^iplnuiophoren von Nizza," 

 Archiv. fur Natvr([?sch, 1854. C. Glaus, "Uebcr Halistenmia tergestimim 

 n. s. nebst Bemerkungen iiber den feineren Bau der Physophoriden," Arl>i-iti'>i 

 aus dem Zoologisclien In.it it tit. der Univ. Wicn. etc., Tom. I., 1878. E. ?I< !- 

 schnikoff, " Studien uber die Entwickelung der Medusen und Sipho.nophoren," 

 Zeitsch.fur nigs. Zool, Tom. XXIV., 1874. 



FIG. 18G. A portion of the stem 

 and appendages of Halistemma 

 tergestimim. St, Stem ; D, hy- 

 drophyllium ; T, dactylozooid; 

 Sf, tentacle of the latter ; If >, 

 female, Ify, male, gonophores. 



