142 MANUAL OF ZOOLOGY. 



often of great length. Besides the tentacles, the margin 01 

 the umbrella is furnished with a series of peculiar bodies, 

 termed " lithocysts," each of which is protected by a sort of 

 process or hood derived from the ectoderm, and consists 

 essentially of a combined " vesicle " and " pigment-spot," such 

 as have been described as occurring separately in the Medu- 

 sidce. These marginal bodies likewise communicate with the 

 chylaqueous canals. The reproductive elements "are lodged 

 in sacciilar processes of the lower portion of the central cavity, 

 immediately above the bases of the radiating canals, and being 

 usually of some bright colour, form a conspicuous cross shining 

 through the thickness of the disc " (Greene). 



By a series of elaborate experiments, Mr G. J. Romanes has 

 shown that the contractions of the swimming-bell of the Sea- 

 blubbers obey the same laws as the contractions of muscular 

 tissue generally, being excitable by the same stimuli, and simi- 

 larly affected by chemical reagents. The removal of the 

 margin of the umbrella causes a more or less total paralysis, 

 and the same effect is produced by the excision of the mar^ 

 ginal bodies, showing that these parts are the seat of the energy 

 by which the movements are effected. Moreover, the severed 

 margin of the swimming-bell continues to contract rhythmically 

 for a considerable time after its complete separation from the 

 organism itself. We are therefore justified in concluding that 

 the margin of the swimming-bell contains what is functionally, 

 if not structurally, a nervous system, though we have at present 

 no direct anatomical evidence to demonstrate this. 



In the RhizostomidcB the reproductive zooids (figs. 60, 6 1), 

 differ from those we have just described as occuring in the first 

 section of the Pelagidce, in not possessing tentacles on the 

 margin of the umbrella, and in having the simple central 

 polypite replaced by a composite dendriform process, which 

 bears numerous polypites, projects far below the umbrella, 

 and is thus described by Professor Huxley : " In the Rhi- 

 zostomidce (figs. 60, 61), a complex, tree -like mass, whose 

 branches, the ' stomatodendra,' end in, and are covered by, 

 minute polypites, interspersed with clavate tentacula, is sus- 

 pended from the middle of the umbrella in a very singular 

 way. The main trunks of the dependent polypiferous tree, in 

 fact, unite above into a thick, flat, quadrate disc, the ' synden- 

 drium,' which is suspended by four stout pillars, the ' dendro- 

 styles,' one springing from each angle, to four corresponding 

 points on the under surface of the umbrella, equidistant from 

 its centre. Under the middle of the umbrella, therefore, is a 

 chamber, whose floor is formed by the quadrate disc, whilst its 



