GNIDARIA ORGANISATION OF SIPHONOPHORA 



109 



upper end of the stem, and represents a metamorphosed Medusa 

 umbrella. (It is wanting only in the order of Calyconecta, where 

 the umbrella of the larval mother 

 Medusa develops into the first pro- 

 visional swimming -bell and is then 

 thrown off.) At one point of the 

 exumbrella an invagination forms at 

 an early stage for secreting air, the 

 air vesicle, which expands so much 

 that it represents by far the largest 

 portion of the original disc ; it always 

 remains in open communication with 

 the exterior by means of the aperture 

 of the invagination (the pore of the 

 air vesicle). Around the air vesicle, 

 in the bell which has been so much 

 modified and has become more or 

 less globular, there are 8 (less fre- 

 quently 4 or 1 6) endodermal chambers 

 divided by septa ; these open under 

 the air vesicle into each other and 

 into the endodermal axial canal of the 

 stem. These chambers correspond 

 with the radial canals of the Medusa. 

 The pneumatophore serves as a hy- 

 drostatic apparatus, which keeps the 

 whole Siphonophora colony floating in 

 the water. The air can be expelled 

 through the pore of the air vesicle, 

 and again secreted by the ectodermal 

 glandular epithelium at its base. 



B. Only one of the tentacles is 

 fully developed. This is moved from 

 the margin of the disc on to the 

 subumbrella to the base of the stem, 

 and is probably usually thrown off at 

 an early stage. 



C. The stem of the Siphonanth, 

 which is generally long, tubular, and 

 contractile, more rarely short and 

 flat, answers to the gastric peduncle 

 of a Medusa. An aperture (primary 



oral aperture) is but rarely found at its lower end. The view that 

 these three parts together are equivalent to a Medusa is supported by 

 ontogenetic observation, as is to a certain extent evident from what has 

 already been said. The gastrula which develops from the fertilised egg 

 grows into a Siphonanth larva, such a larva possessing at first only 



sb, Pneumatophore ; sg, swimming-bell ; ds, 

 bract ; t, tentacles ; groj, go?, go$, gonophores ; 

 hy, oral or gastric peduncle (siphon) ; p, 

 feeler or taster (palpon) ; A-H, various 

 groups of appendages which are never found 

 in this way together in any single Siphon- 

 anth. Black portion = gastric system. 



