124 HTDEOZOA. 



ture ; so that the relation between the producing 

 and produced zooid is here by no means the same 

 as in the other orders of Hydrozoa, The true 

 import of this fact should not escape attention. 



All the Lucernaridce may be at once distin- 

 guished by their umbrella. The cup or disc in 

 the Lucernariadce and Hydra-tubae, the swimming 

 organ of Pelagia and of the free zooids, are alike 

 included under this designation. A free umbrella 

 differs from a nectocalyx, with which it is often 

 confounded, (i), in the absence of a veil; (2), in 

 its mode of development ; and ( 3 ), in the nature of 

 its canal system and marginal bodies. The ra- 

 diating canals, never less than eight in number, 

 send off numerous anastomosing branches, which 

 form a very intricate net-work. The peculiar 

 structure of the lithocysts has been previously 

 explained. Each is supported on the end of a 

 short double-walled stalk, the cavity of which 

 runs into one of the radiating canals. Protection 

 is given to this apparatus by a hood-like, cres- 

 centic fold of the ectoderm, at the base of which, 

 and on the convex surface of the umbrella, a funnel- 

 shaped orifice has been observed, whereby the ra- 

 diating canal communicates with the exterior. 

 Apertures similar in function, but not in position, 

 have been met with by Mr. Huxley in certain of 

 the Medusidce, There are no lithocysts in the 

 Lucernariadce^ unless the simple tubercles, placed 

 between the tentacular tufts, on the margin of 

 Liicernaria auricula, be regarded as these organs 

 in a rudimentary condition. 



The LucernaiHdce manifest another characteristic 

 feature in their gastric filaments, the presence of 

 ■svhich appears to be universal throughout the 



