142 ZOOLOGY SECT. 



Order 2. — Trachylin^. 



Hydrozoa in whifli no fixed zoophyte stage is known to occur, 

 all members of the group being locomotive medusse, some of which 

 have been proved to develop directly from the egg. The sense- 

 organs are formed partly of endt)derm. 



Sub-Order a. — Trachymedusw. 



Trachylinse in which the tentacles spring from the margin of the umbrella, 

 and the gonads are developed in connection with the radial canals. 



Sui-Ordcr b. — Narcomeduscc . 



Trachylina! in which the tentacles spring from the ex-umbrplla, some dis- 

 tance from the margin, and tlie gonads are developed in connection with the 

 manubrium. 



Order 3. — Hydrocorallina. 



Hydrozoa in which a massive skeleton of calcium carbonate is 

 secreted from the ccenosarc, 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 Paleeozoic 

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



Systeynatic Positioyi of the Example. 



Obelia, in virtue of the possession of gono- and hydrothecse, 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^. 



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. 



Creneral 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 



