28 CCELEXTEKATA. 



To the form of disk without a velum Huxley restricts the term 

 umbrella : in the Calycozoa it is prolonged aborally into a longer 

 or short peduncle, terminating in a hydro rhiza, by which the 

 animal is enabled to fix itself to any foreign body at will. When 

 detached, the contractions of the umbrella enable it to swim with 

 the ease of an ordinary medusoid body. The order contains but 

 one family : 



Lucernariidce. 



Lucernaria. 



Depastrum. 



Carduella. 



Subclass HI. SIPHON OPHOKA. 

 OCEANIC HYDROZOA. 



Hydrosome free and oceanic, simple or branched, consisting of 

 several polypites connected by a contractile coenosarc, and attached 

 at the proximal end to a disk, float [pneumatophore], or body-sac 

 [somatocyst]. 



The polypites have each a tentacle, often of great length, pro- 

 vided with lateral branches having thread-cells [trichocysts, mo- 

 dified zooids]. Each polypite is occasionally protected at the 

 base by overhanging plates [hydrophyllia]. Certain bell-shaped 

 cups [specialized nectocalyces] are frequently present, attached 

 to the hydrosorne, by the contraction of which the animal is 

 propelled through the water. The pneumatophore contains an 

 air-sac [pneumatocyst], variously formed, with firm chitinous 

 walls. Vesicles and pigment-spots [ocelli], often very brilliant, 

 are found round the margins of the nectocalyces : the former 

 have been called " otolites," and have been supposed to be audi- 

 tory organs ; the latter are possibly the earliest indication of 

 eyes. 



The Siphonophora are organisms of a very delicate and pecu- 

 liarly composite character, almost exclusively found floating on 

 the surface of tropical seas. They have rarely a radiate character, 

 but are either bilateral or unsymmetrical. Their reproductive 

 organs are gonophores, varying from the simplest form to me- 

 dusoids of the normal type. 



A body-sac at the proximal end CALYCOPHOR.E. 



A float at the proximal end Pn YSOPHORX. 



