COELENTERATA 59 



well armed with nematocysts. (v.) Nectocalyces : swimming 

 bells. They consist of the umbrella of a medusoid, with four 

 radial canals, but without a manubrium or mouth, (vi.) Pneu- 

 'mato'phons : 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 (Velella), 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 being occasionally found on our 

 shores. 



The two remaining orders of the . HydromeduSae — ithe 

 Tkachomedusae and the Naecomedusae — 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 Juive their tentaculocysts 

 free or enclosed in capsules. Coecal radial cancels may he 

 present, which open into the circular canal, but never reach 

 the central stomach. 



