106 PR AY A DIPHYS. PART m. 



In the Praya, the groups are individualized in the 

 highest degree consistent with union ; for, when the 

 animal is at rest, each of the individual groups, amount- 

 ing to thirty or forty, swims about by means of its little 

 bell independent of the rest. Their motions can be com- 

 pared to nothing but a troop of jugglers performing 

 gymnastic exercises round a cord represented by the 

 common body of the animal ; except for adherence to 

 which the life and will of each group are so perfectly in- 

 dependent, that the mutual dependence of the whole is 

 only seen when the common trunk contracts to bring 

 all its appendages towards the two principal bells, which 

 then begin to move. 7 



Thus each group has a special life and motion, con- 

 trolled by a general life and motion ; strong individual 

 muscular power controlled by general muscular power ; 

 yet no nervous system has as yet been discovered, so 

 this animal activity must for the present be attributed 

 to a strong, inherent, contractile power in the muscular 

 fibre. The Praya is seldom complete, on account of 

 the ease with which it casts off its great bells. 



None of the Diphyidse have special organs for respira- 

 tion ; their juices are aerated through their delicate 

 tissues. They are dioecious, and invariably produce 

 perfect male and female medusiform zooids; they are 

 situated among the groups of the polypites and their 

 appendages, and are attached to the axis of the animal. 

 When free, they swim away by the contraction of their 

 bells ; the eggs are fertilized, and produce young Di- 

 phyidse, male and female ; so these animals, like most 

 of the oceanic Hydrozoa, have two alternate stages 

 of existence. 



7 'Recherches sur quelques Ammaux inferieurs de la Mediterranee,' par 

 C. Vogt : Memoires de I'Institut National Genevois, torn. i. 



