SIPHONOPIIORA 



299 



and tlioir function is, by rhytlnnic contractions, to pn)})cl the 

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



Gastrozooids. — These are tubular or saccular zooids ])rovi(lcd 

 with a nioutli and attached by their aboral extremity to tlie 

 stolon (Fig. 142, (.1). 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 otliers 

 it is dilated to form a trumpet-like lip. 



Dadylozooids. — In Velella and Porjnta the daetylozooids are 

 similar in general cliaracters 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 

 Si[)honophora, how- 

 ever, the daetylozooids 

 are very long and 

 elaborate filaments, 

 which extend for a 

 great distance from tlu; 

 colony into the sea. 

 They reach their most 

 elaborate condition in 

 the Calycophorac. 



The dactylozooid 

 in these forms has a 

 hollow axis, and the 

 lumen is continuous 

 with the cavity of the 

 neigh])ouring gastrozooid, 



Fig. 1-11. — A small Crustacean {Rhinocolanvs) caught 

 l)y a terminal filament (f.t) of a liattery of Ste2ihu)M- 

 p/ii/cs. b. The pio.xinial end of the battery with the 

 most powerful nematocysts ; e, elastic l>anil : ^'. 

 stalk supporting the hattery on the daotylo/.oniil. 

 (After Chun.) 



Arranged at regular intervals on the 

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



