124 THE OCEAN WORLD. 



of the corolla fade, fall, and die, and a bud replaces them, which 

 produces a new polyp \ and so on. This succession determines 

 the length of the stem. Each apparent flower throws out a small 

 tube, which terminates it, and each addition adds one joint more 

 to the axis, which it increases in length. 



SERTULARIAD.<E. 



All the Hydrozoa, with the exception of Hydra and a few other 

 genera, are marine productions, varying in size from a few lines to 

 upwards of a foot in height. The members of this order are found 

 attaching themselves to rocks, shells, seaweeds, and corallines, and to 

 various species of Crustacea. Many of them attach themselves indis- 

 criminately to the nearest object, but others show a preference for some 

 special substance. Thtciaria thuia attaches itself to old bivalves ; 

 Thoa halednia prefers the larger univalves ; Antennularia antennina 

 attaches itself to coarse sand or rocks ; Laomedea geniculata delights 

 in the broad frond of the tangle ; Plumularia catherina attaches itself 

 in deep water to old shells, corallines, and ascidians, growing in a 

 manner calculated to puzzle the naturalist, as it did Crabbe, the poet, 

 who writes of it : 



" Involved in sea-wrack, here you find a race 

 Which science, doubting, knows not -where to place ; 

 On shell or stone is dropp'd the embryo seed, 

 And quickly vegetates a vital breed." 



Sertularia pumila, on the other hand, loves the commoner and coarser 

 wrack. " The choice," says Dr. Johnston, " may in part be depen- 

 dent on their habits, for such as are destined to live in shallow water, 

 or on a shore exposed by the reflux of every tide, are, in general, 

 vegetable parasites ; while the species which spring up in deep seas 

 must select between rocks, corallines, or shells." There seems to be 

 a selection even as to the position on the rocks. According to 

 Lamouroux, some polyps always occupy the southern slopes, and 

 never that towards the east, west, or north ; others, on the contrary, 

 grow only on these exposures, and never on the south, altering their 

 position, however, according to the latitude, and its relation to the 

 Equator. 



The Sertularidce have a horny stem, sometimes simple, sometimes 

 so branching that they might readily enough be mistaken for small 

 plants, their branches being flexible, semi-transparent, and yellow. 

 Their name is derived from Sertum, a bouquet. Each Sertularia has 



