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 
t> the axis, which it increases in length. 
SERTULARIADA, 
All the Aydrozoa, 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. TZhuéaria thuia attaches itself to old bivalves ; 
Thoa haleinia 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 ona 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 Sertudaride 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 Serum, a bouquet. Each Sertularia has 
