H 



THE INVERTEBRATA 



coenosarc : they are lodged in pits of the skeleton called ampullae, but 

 their liberation has been observed in Millepora. It is supposed, 

 however, that their free-living existence is very brief. 



Order SIPHONOPHORA 



The Siphonophora are colonial animals which exhibit the maximum 

 development of polymorphism found in the Coelenterata or indeed 

 in any group of the Animal Kingdom. They are pelagic and each 

 colony originates from a planula which metamorphoses to form a 

 single individual, from the base of which springs a coenosarcal tube 

 budding off all the other members of the colony. It usually happens 



,amp. 



':icanj 



^can.2 



tdh. 



Fig. 1 20. Diagrammatic section through Millepora showing a gastrozooid 

 with a dactylozooid (dact.) on each side of it and an ampulla (amp.) with a 

 medusa enclosed in it; can. i, the living canals, shown in black, and can. 2, the 

 degenerating canals, shown as lines, constitute the hydrorhiza, and the skeleton 

 is represented by stippling; med. a medusa just liberated; tab. tabulae in a 

 gastropore. Slightly altered from Hickson. 



that those which are developed first are needed to buoy up and propel 

 the young colony. Consequently the first individual is either medusi- 

 form or else forms an apical float or pneumatophore (the epithelium 

 of which secretes gas). There may also be formed from the ectoderm 

 of the first formed individual an oleocyst containing a drop of oil. 

 The medusiform individuals resemble the bell of an anthomedusa, 

 with velum and canal system but lacking the manubrium, and they 

 are called nectocalyces : while the most primitive siphonophores have 

 only a single one there may be a series of them. Following these the 

 coenosarc in one type of colony (Fig. 121 A) grows to a great length 

 and buds off at intervals along its length similar assemblages of 



