HYDROZOA 169 



phore consists of a chitinous disc containing a number of chambers 

 and raised into a vertical ridge which forms a sail. On the under 

 surface there is a single large gastrozooid in the centre, a larger 

 number of gonozooids surrounding it and a fringe of dactylozooids 

 at the margin. The gonozooids produce buds which actually escape 

 as free medusae. The coenosarc consists of a mass of tissue which is 

 traversed by endodermal tubes placing in communication the cavities 

 of the gastrozooid and the gonozooids, and ectodermal tubes (tracheae) 

 which are prolongations of the gas cavity of the pneumatophore. This 

 tropical form is often brought in large numbers to the shores of 

 Devon and Cornwall by the Gulf Stream. 



The medusae and nectocalyces of the Siphonophora are very 

 similar to the Anthomedusae. Medusae like Sarsia (Fig. 124C) may 

 bud off other medusae either from the bell or the manubrium, but 

 the Siphonophora are probably not to be regarded simply as a colony 

 of medusae connected by coenosarc. A further change has gone on 

 in which organs have been displaced from their original position. The 

 manubrium has come to lie outside the primary medusa bell, forming 

 a gastrozooid (Fig. 124 B, gst.) at the beginning of the main coeno- 

 sarcal axis. No manubria corresponding with the medusa bells of the 

 nectocalyces are present. In the cormidia the hydrophyllium which 

 may be a modified bell, the gastrozooid and the tentacles may be 

 quite separate from one another while the complete medusoid form 

 is shown only by the fixed gonophores (Fig. 124 A and B). 



In more specialized siphonophores owing to the shortening of the 

 main axis the displacement of parts is more extreme and the com- 

 ponent parts of the cormidia no longer recur in the typical groups, 

 all kinds of organs being crowded together. Lastly, with the great 

 development of the gas-secreting pneumatophore, the medusa bell 

 is suppressed. 



While the above description gives an impression of the order re- 

 garded as colonial animals the siphonophores must be primarily 

 considered as coelenterates exhibiting growth variability to such an 

 extent that the identification of the component structures as organs 

 or individuals is difficult and of purely academic interest. 



Order GRAPTOLITH INA 



Extinct, probably planktonic, animals ; if related to the Hydrozoa, the 

 polyp generation is dominant, the medusoid generation unfossilized 

 or possibly represented by the prosicula; the individuals are budded 

 oflF from one another and remain in contact with the parent ; there is 

 no definite coenosarc; and the perisarc is produced round the polyps 

 as hydro thecae. 



