108 



. Oaelentera. 



latter. (2) Swimming bells (nectocalyces) ^ sessile and sterile 

 medussSj without manubrium or moutlij but with well-developed 

 umbrella and velum, by the contractions of which the colony moves 

 about. (3) Covering pieces (hydrophyllia) , like the necto- 

 calyceSj but with the umbrella reduced to a stiff plate ; they serve 

 •as coverings for other individuals. 



Fig. 62. 



Fig. 63. 



Fig. 62. Biagram of a Siphonophoran with 

 -V an elongated hydrosorua (Physophora) ; the 

 ^ ^ digestive cavities are drawn black. — Orig. 



Fig. 63. Diagram of a Siphonophoran with 

 a disc-like hydroaoma (Porpita). — Orig. 



General lettering : d covering piece, / ten- 

 tacle, I air sac, mf fertile Medusae, N large 

 Gastrozooid, n, small ditto, p disc with small 

 air cavities, s swimming bell, ta feeler, te 

 teutacle-like person. 



A colony of the Siphonophora consists of these individuals, but 

 they are not always all present ; the swimming bells may for instance 

 be absent, in which case the colony drifts passively; the covering 

 pieces may also be wanting; the number and arrangement of the 

 individuals, and consequently the form of the colony, is extremely 

 varied. 



The Siphonophora are true pelagic animals, are almost ex- 

 clusively limited to tropical and warm seas [e.g., frequent in the 

 Mediterranean) . 



As examples may he m.eiitioned : Physophora and its allies (Kg. 62), -with 

 long hydi'OSQma, terminating in a small pneumatophore, and -with numerous 

 nectocalyces on the upper part of tlie stem ; Physalia vpith huge pneumatocyst 

 and bearing gastrozooids and hydrocysts (with long tentacles), without hydro- 

 phyllia or nectocalyces ; Porpita (Fig. 63) with disc-like circular hydrosoma 



