COELENTERATA 59 
well armed with nematocysts. (v.) Vectocalyces: swimming 
bells. They consist of the umbrella of a medusoid, with four 
radial canals, but without a manubrium or mouth. (vi.) Pneu- 
matophores: these form floats, and the air secreted within 
them serves to keep the whole colony in a vertical position. 
(vii.) The sexual persons: these may become free medusae of 
the craspedote type—that is, provided with a velum; or they 
may remain undeveloped as sporosacs. The medusae have their 
generative organs on the radial canals (Velel/a), or on the 
manubrium (Physalia). The colonies are usually herma- 
phrodite. 
The seven modifications described above are not all to be 
met with in every Siphonophoran, but usually four or five of 
them coexist in each colony, thus giving rise to a form of 
extreme complexity ; various combinations of these persons also 
permit great variety amongst the different species. Many of 
them are brilliantly coloured in parts, and are amongst the 
most beautiful marine objects which float along the surface of 
the sea. They are common in the Mediterranean and open 
seas, Velella and Diphyes bemg occasionally found on our 
shores. 
The two remaining orders of the HypromMEDUSAE—the 
TRACHOMEDUSAE and the NARCOMEDUSAE—have no hydroid 
stage, but the ova in most cases give rise immediately to the 
medusae. They are characterised by the possession of sense 
organs formed by modifications of the tentacles on the rim of 
the umbrella. They are termed tentaculocysts, and into them 
alone of the sense organs of the Hydromedusae does the endo- 
derm enter. One or more otoliths formed from the endoderm 
cells, which correspond with the axis of the ordinary tentacle, 
contain crystalline concretions, and form the auditory organ. 
Order 5. Trachomedusae. 
CHARACTERISTICS.— The Trachomedusae have their tentaculocysts 
Sree or enclosed in capsules, Coecal radial canals may be 
present, which open into the circular canal, but never reach 
the central stomach. 
