SIPHONOPIIORA 



299 



and their function is, by rhythmic contractions, to propel the 

 colony through the water (Fig. 142, N). 



Gastrozooids. — These are tubular or saccular zooids provided 

 with a naouth and attached by their aboral extremity to the 

 stolon (Fig. 142, G). In some cases the aboral region of the 

 zooid is differentiated as a stomach. It is dilated and bears the 

 digestive cells, the oral extremity or hypostome being narrower 

 and more transparent. In some cases the mouth is a simple 

 round aperture at the extremity of the hypostome, but in others 

 it is dilated to form a trumpet-like lip. 



Dactylozooids. — In Velella and Porpita the dactylozooids are 

 similar in general characters to the tentacles of many Medusae. 

 They are arranged as a 

 frill round the margin 

 of the colony, and 

 each consists of a 

 simple tube of ecto- 

 derm and endoderm 

 terminating in a 

 knobbed extremity 

 richly provided with 

 nematocysts. 



In many other 

 Siphonophora, how- 

 ever, the dactylozooids 

 are very long and 

 elaborate filaments, 

 which extend for a 

 great distance from the 

 colony into the sea. 

 They reach their most 

 elaborate condition in 

 the Calycophorae. 



The dactylozooid 

 in these forms has a 

 hollow axis, and the 

 lumen is continuous 

 with the cavity of the 

 neighbouring gastrozooid. Arranged at regular intervals on the 

 axis is a series of tentacles (" tentilla "), and each of these supports 



Fig. 141. — A small Crustacean (PMnocalanus) caught 

 by a terminal filament (f.t) of a battery of Stephuno- 

 phyes. b, The proximal end of the battery with the 

 most powerful nematocysts ; e, elastic band ; S, 

 stalk supporting the battery on the dactylozooid. 

 (After Chun.) 



