Chap. III.] 



JELLY-FISHES. 



In the third group 

 we have those free- 

 swimming colonies to 

 which reference has 

 already been made as 

 examples of the highest 

 form of division of 

 labour : they are called 

 the Siphonophora, 

 and Velella, Diphyes, 

 Physalia, and Physo- 

 phora (Fig. 12) belong 

 to this group. 



Scypliomedusse. 

 In the second great 

 division of the hydrozoa 

 we have the forms 

 which are best known 

 as the Medusae, or jelly- 

 fishes par excellence. 

 With one exception, 

 they all pass through 

 a stage which, at first 

 somewhat hydriform in 

 appearance (Scypliis- 

 toma-stage), is re- 

 markable for under- 

 going transverse di- 

 vision ; each of the 

 segments so formed 

 separates and forms an 

 independent medusa. 

 When adult they are 

 always medusiform in 

 appearance, and, as they 

 rarely have a velum to 

 their disc, they are 



Fig. 12. PhysopTiora hydrostatica. 



a, Air-bladder; m, nectocalyx ; jjr, gener- 

 ative persons; ?., nutrient, persons (in 

 the form of sucking tubes) ; t, tentacu- 

 lar persons. (After Cuvier.) 



