GYMNOBLASTEA ANTHOMEDUSAE 265 



uncoiling themselves ; they have a small mouth and a single 

 circlet of rudimentary tentacles. The " tentaculozooids " are 

 situated at the outskirts of the colony, and are very long 

 and slender, with rudimentary tentacles and no mouth. The 

 " blastostyles," usually shorter than the gastrozooids, have two 

 circlets of rudimentary tentacles and a mouth. They bear on 

 their sides the spherical or oval gonophores. 



The medusome stage in the life-history of these Hydrozoa is 

 produced by gemmation from the hydrosome, or, in some cases, 

 by gemmation from the medusome as well as from the hydro- 

 some. In many genera and species the medusome is set free as 

 a minute jelly-fish or Medusa, which grows and develops as an 

 independent organism until the time when the sexual cells are 

 ripe, and then apparently it dies. In other Gymnoblastea the 

 medusome either in the female or the male or in both sexes does 

 not become detached from the parent hydrosome, but bears the 

 ripe sexual cells, discharges them into the water, and degenerates 

 without leading an independent life at all. In these cases the 

 principal organs of the medusome are almost or entirely function- 

 less, and they exhibit more or less imperfect development, or 

 they may be so rudimentary that the medusoid characters are no 

 longer obvious. Both the free and the undetached medusomes 

 are gonophores, that is to say, the bearers of the sexual cells, but 

 the former were described by Allman as the " phanerocodonic " 

 gonophores, i.e. " with manifest bells," and the latter as the 

 " adelocodonic " gonophores. The gonophores may arise either 

 from an ordinary zooid of the colony {Syncoryne), from a specially 

 modified zooid — the blastostyle — as in Hydractinia, or from the 

 hydrorhiza as in certain species of Perigonimus. The free- 

 swimming Medusa may itself produce Medusae by gemmation 

 from the manubrium {Sarsia, Lizzia, Rathkea, and others), from 

 the base of the tentacles (Sarsia, Corymorpha, Hyhocodov), or 

 from the margin of the umbrella {Eleutheria). 



The free-swimming Medusae or phanerocodonic gonophores 

 of the Gymnoblastea are usually of small size (1 or 2 mm. in 

 diameter) when first liberated, and rarely attain a great size even 

 when fully mature. They consist of a circular, bell-shaped or 

 flattened disc — the umbrella — provided at its margin with a 

 few or numerous tentacles, and a tubular manubrium bearing 

 the mouth depending from the exact centre of the under (oral) 



