SECT. IV 



PHYLUM CCELENTERATA 



161 



fin, 



net 



seas, and consists of a long, slender, floating stem, to which a 

 number of structures, differing greatly in form, are attached. At one 

 the uppermost end of the stem is an ovoid, bubble-like body con- 

 taining air the float or pneumatophore (pn). Next come a number 

 of closely set, transparent structures (net), having the general char- 

 acters of unsymme- 

 trical medusae with- 

 out manubria, each 

 being a deep, bell- 

 like body, with a 

 velum and radiating 

 canals. During life 

 these swimming- 

 bells or neclocalyces 

 contract rhythmic- 

 ally i.e. at regular 

 intervals drawing 

 water into their 

 cavities, and imme- 

 diately pumping it 

 out, thus serving to 

 propel the entire 

 organism through 

 the water. Below 

 the last nectocalyx 

 the character of the 

 structures borne by 

 the stem changes 

 completely : they 

 are of several kinds, 

 and are arranged 

 in groups which 

 follow one another 

 at regular intervals, 

 and thus divide the 

 stem into segments, 

 like the nodes and 

 internodes of a 

 plant. 



Springing from 

 certain of the 

 " nodes " are un- 

 mistakable polypes 

 (p), differing however from those we have hitherto met with in 

 having no circlet of tentacles round the mouth, but a single long 

 branched tentacle (t) arising from the proximal end, and bearing 

 numerous groups or " batteries " of stinging-capsules (ntc). In 



VOL. I M 



FIG. 121. Diagram of a Siphonophore : the thick line repre- 

 sents endoderm ; the space external to it, ectoderm ; the 

 internal space, the enteric cavity, coe. coenosarc ; dz. dactylo- 

 zooid ; hph. hydrophyllium ; md. sporosac ; net., net'. 

 nectocalyces ; ntc. battery of nematocysts ; p. polype ; pn. 

 pneumatophore ; t. tentacle. (After Claus.) 



