166 



ZOOLOGY 



SECT. 



gastrozooid (%). 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 

 medusae with manubrium, 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. 124. Porpita pacifica. A, from beneath ; B, vertical section, hij, large central gastro- 

 zooid ; hy'. blastostyles ; sh, chambered shell ; t, dactylozooids. (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 i.e. to consider them as hydroid colonies in 

 which the cosnosarc is represented by the discoid or rhomboid 

 body 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 

 trace of the soft parts having, as in the majority of fossils, disappeared. 



With one doubtful exception they are compound, consisting of an elongated 

 tube the perisarc of the common stem, having attached to it, either in a single 



