PRIMITIVE ANIMAL FORMS 119 



was the first to describe the whole of the small and varied 

 world in Hydractinia that encrusts the shells inhabited by the 

 Hermit-crab. This variety of associated forms, which often 

 corresponds to that presented by the leaves of a plant, is 

 widely spread among the Hydroids, and has led to the same 

 results. When a gamomerid, or sexual merid, develops, at any 

 particular part of the hydrozoid, it brings about the trans- 

 formation of the neighbouring merids into dactylomerids. 

 All of these form a single whorl, which admits four merids, 

 for the simple reason that any circumference offers about 

 three times as much room as its diameter, and very little more. 

 These dactylomerids cannot coil about the gamomerid they 

 surround without drawing towards them the periphery of the 

 peduncle upon which they grow, and they thus necessarily 

 form a bell-shaped web membrane of which the gamomerid 

 constitutes the clapper. The walls contain muscles which 

 permit it to contract and instantly drive out the water which 

 fills it, and the recoil produced by this sudden expulsion of 

 water has the effect of tugging at the support which at its 

 summit unites it to the Hydrozoid. The peduncle finally breaks 

 and the bell is set free. It consists of a gamomerid provided 

 with a mouth and capable of digestion, an umbrella, which serves 

 as an organ for swimming, and four fishing-tentacles — all that 

 is necessary for maintaining an independent existence. It is 

 free henceforth to live in its own way. A new type of organism, 

 a veritable flower-animal, has come into being ; this flower- 

 animal is known as a medusa. The medusa? may remain 

 attached to the hydrozoid that produced them, which then 

 becomes a hydrodeme, since the creatures are themselves all 

 hydrozoids. Their formation is frequently influenced by 

 tachygenesis ; hence they sometimes remain incomplete. 



Instead of attaching themselves to some solid body, certain 

 hydromerids, drawn by their lightness to the surface of the 

 water, find a way of imprisoning an air-bubble which thence- 

 forward buoys them up. The hydrodemes resulting from their 

 development remain floating and act together, like a fish 

 pursuing and capturing prey. Gastromerids, dactylomerids, 

 and medusae then take on the most diverse forms. A certain 

 number of contiguous medusae are employed like a crew of 

 oarsmen for locomotion, and by a phenomenon of tachygenesis 

 these medusae, which have an indispensable function, even 



