154 



ZOOLOGY 



SECT. 



which is the only nutritive zooid, taking in food for the entire 

 colony. The closely allied genus Velclla is of rhomboidal form, 

 and bears on its upper surface an oblique sail. 



The reproductive zooids are liberated as free medusae. The 

 eggs give rise to young which have a close resemblance to flat 

 medusa? with maiiubrium, marginal tentacles, and an air-chamber 

 or float developed in the ex-umbrella. Thus it is quite possible 

 that the Siphonophora of the Porpita-type may be medusae the 

 sub-umbrella of which has given rise to buds forming the feelers 



FIG. 111. Porpita pacifica. A, from beneath; B, vertical section; 7i.;/. polype ; Inf. dactylo- 

 zooids ; sit. chambered shell; t. tentacles. (From Parker's Biology, after Duperry and 

 Koelliker.) 



and blastostyles. But, as their early development is not known, it 

 is still quite legitimate to describe them in the same terms as the 

 other Siphonophora /.!-'. to consider them as hydroid colonies in 

 which the coanosarc is represented by the discoid or rhomboid 

 bodv with its contained air-chamber. 



ORDER 5. GRAPTOLITHIDA. 



The " Graptolites " are fossil Hydrozoa found in the Upper Cambrian and 

 Silurian rocks. They are known only by their fossilised chitinoid skeleton, all 

 tract- of tlu- soft parts having, as in the majority of fossils, disappeared. 



