THE HYDROMEDUSAE 



39 



medusoid, and a metagenetic life-history. Gonophores rarely 

 freed, generally sessile. 



The Siphonophora (for the literature of which Haeckel, 39 ; 

 Chun, 40, 41 ; and Schneider, 42 ; should be consulted) are invari- 

 ably free-swimming, colonial, and polymorphic. Just as the planula 

 in some Anthomedusae does not 

 itself develop into a hydroid, 

 but becomes a budding hydro- 

 rhiza, so in all probability, in 

 this group (a part at least of), 

 the planula is to be regai'ded 

 as itself giving origin to the 

 coenosarc, and as budding 

 numerous individuals of vary- 

 ing form and function. The 

 composition of the colony is 

 very different in the different 

 families, but is generally a 

 combination of some of the fol- 

 lowing hydroid or medusoid 

 individuals. 



POLYMORPHIC MODIFICA- 

 TIONS OF THE HYDROID : 



1. The gastrozooid (siphon, 

 polypite) has a large mouth, 

 and is provided with nemato- 

 cysts ; at or near its base is 

 usually placed a single tentacle 

 (Figs.44,e; 51,). Thetentacle 

 is generally extremely long and 

 contractile ; it is tubular, and is 

 either itself provided with bat- 

 teries of nematocy sts ( Apolemia), 

 or bears a large number of fine 

 lateral threads or tentilla, carry- 

 ing numerous nematocysts 

 (Forskalea). The latter can 



sometimes be spirally retracted into a protective cup or involucrum 

 (Agalmopsis). In some cases no tentacle is developed (Velella). 

 The endoderm of the gastrozooid is generally pigmented, and often 

 projects as villi into the coelenteron. 



2. The dactylozooid (hydrocyst, palpon) is generally devoid of 

 a mouth, and provided liberally with nematocysts. The palpade 

 or tentacle of the dactylozooid is never branched, and generally 

 grows on or near the base (Figs. 44, g, h ; 51, D, T). No tentacle is 

 developed in some cases (Velella). To such an extent are the 



FIG. 44. 



44. Diagram showing possible modifica- 

 tions of medusoids and hydroids of a colony 

 of Siphonophora. n, pneumatophore ; k, necto- 

 phores (swimming bells) ; I, hydrophyllium 

 (covering piece) ; i, generative medusoid ; g, 

 dactylozooid with attached tentacle, h ; e, gas- 

 trozooid, with branched grappling tentacle, 

 /; m, stem or conn. The thick black line 

 represents endoderm, the thinner line ectoderm. 

 (After Allman.) 



