PHYLUM CCELENTERATA 143 



ORDER 3. HYDROCORALLINA. 



Hydrozoa in which a massive skeleton of calcium carbonate is 

 jreted from the coenosarc, the dried colony being a coral. 



ORDER 4. SIPHONOPHORA. 



Pelagic Hydrozoa in which the colony usually exhibits extreme 

 polymorphism of its zooids. 



ORDER 5. GRAPTOLITHIDA. 



An extinct group of Hydrozoa, found only in rocks of Palaeozoic 

 age, in the form of the fossilised perisarc of the branched colonies. 



Systematic Position of the Example. 



Obelia, in virtue of the possession of gono- and hydro-thecae, and 

 of gonads formed in connection with the radial canals, belongs to 

 the sub-order Leptomedusse. It is placed in the family Campanu- 

 lariidce, distinguished by having cup-shaped thecae borne at the 

 ends of distinct branchlets : the genus Obelia is distinguished 

 from other genera of the same family by the fact that the 

 reproductive zooids are free-swimming medusae. 



ORDER 1. LEPTOLIN.E. 



The more typical members of this group agree in all essential 

 respects with Obelia, consisting of branched colonies bearing two 

 principal forms of zooids, which serve for nutritive and reproductive 

 purposes respectively. 



General Structure. The form and size of the colonies are 

 subject to great variation : they may be little insignificant tufts 

 growing on shells, sea- weeds, &c., or may take the form of com- 

 plex trees three feet in height, and containing many thousand 

 zooids. The hydranths may be colourless and quite invisible to 

 the naked eye, or, as in some Tubulariae (Fig. 106, <5), may be bril- 

 liantly coloured, flower-like structures, nearly an inch in diameter. 

 The medusae may be only just visible to the naked eye, or, as in 

 dZquorea, may attain a diameter of 380 mm., or about 15 inches : 

 they are often seen with great difficulty owing to the bubble-like 

 transparency of the umbrella ; but frequently the manubrium is 

 brightly coloured, or brilliant dots of colour the ocelli or eye-spots 

 may occur around the margin of the umbrella. They are also 

 frequently phosphorescent, the phosphorescence of the ocean being 

 often due to whole fleets of medusae liberated in thousands from 

 the hydroid colonies beneath the surface. 



The two sub-orders of Leptolinae are distinguished by the 

 arrangement of the perisarc. In the Anthomedusae, of which 



