124 HYDROZOA. 



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 Lucemaridce may be at once distin- 

 guished by their umbrella. The cup or disc in 

 the Lucernariadce and Hydra-tubse, the swimming 

 organ of Pelagia and of the free zooids, are alike 

 included under this designation. A free umbrella 



) 



with 



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 

 Lucernariadai, unless the simple tubercles, placed 

 between the tentacular tufts, on the margin of 

 Lucernaria auricula, be regarded as these organs 

 in a rudimentary condition. 



The Lucernaridw manifest another characteristic 

 feature in their gastric filaments, the presence of 

 which appears to be universal throughout the 



