CHAPTER XV 

 OBELIA GENICULATA 



EVERYBODY who has been to the seaside in the summer 

 months is familiar with the large jelly-fishes which often float 

 in millions past the shores. But everybody is not so 

 familiar with the much smaller transparent jelly-fishes, 

 measuring some 3 mm. in diameter, which really are more 

 abundant. They can easily be caught in a fine muslin net 

 towed slowly behind a boat, and they are beautiful little 

 objects when turned into a glass jar or aquarium. So unlike 

 is one of these little jelly-fishes, or Medusae, to a Hydra, that 

 nobody would suspect that it is at once the child and the 

 parent of a hydra-like form, yet such is the case as the 

 present chapter will show. 



The organism known as Obelia geniculata is a little stock 

 or colony formed by the repeated budding of a Hydra-like 

 person produced from the ovum. We have seen in the 

 last chapter how Hydra gives rise to lateral buds, and that 

 sometimes these may produce secondary buds before they 

 drop off from the parent stem. Now if a Hydra were to 

 grow to a considerable length and produce a single bud 

 which did not drop off but lengthened out and in turn pro- 

 duced another bud, and if this secondary bud produced 

 a tertiary and the tertiary a quaternary, and so on, a simple 

 colony of hydriform persons would result, all of whose 

 members would be united together and have their 

 gastro-vascular cavities in connection. This condition is 

 realised in Obelia. There are several species in the genus 

 differing from one another chiefly in their mode of budding, 

 and consequently in the eventual form of the plant-like 

 colonies which result from budding. The species known as 

 Obelia geniculata is at once the simplest and most common 

 of the genus. It is to be found in great abundance just 

 below low- water mark, generally growing on the broad fronds 



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