168 LECTURE IX. 



The tentacula in the British species of bfire-eyed Medusae are 

 simple, usually filiform, and highly contractile. "Each of these 

 organs," writes Prof. E. Forbes, " may be extended or contracted 

 singly, or in concert with its fellows, evidently obeying promptly the 

 will of the animal of which they form part. They guide the Medusa 

 through the sea, and can anchor it. I have seen a Geryonia anchor 

 itself by means of its lips, clasping a coralline with them, and remain- 

 ing tranquil so fixed for a considerable time." * 



The Medusae swim by the contractions of the margin of their disc ; 

 and Hunter has put up a corrugated portion of the under surface of 

 this part of a Rhizostoma f which he considered as indicative of the 

 arrangement of muscular fibres in that part. Subsequent microscopic 

 observations have confirmed the accuracy of his views of this etnic- 

 ture. 



The fibres run parallel with each other, with occasional interrup- 

 tions in their coursej, and their component fibrillce, like those in 

 Oceania and Pelagia noctiluca^, present the transverse striated 

 character of voluntary muscle. Prof. E, Forbes states that he has 

 paralysed one side of a Rhizostoma, whose disc measured more than 

 a foot across, by removing wath a scalpel the subdiscal fibrous bands 

 of that half, whilst the other side contracted and expanded as usual, 

 though with more rapidity, as if the animal was alarmed or suf- 

 fering. || 



The evidence of distinct nerves and ganglions in the pulmograde 

 Acalephce rests, at present, on the interpretation which Ehrenberg has 

 given of certain appearances in connection with the eight brown 

 marginal "cysticles" {d, d,Jig. 76.), which in the Cyancea aurita are 

 made conspicuous by a speck of red pigment on their upper or dorsal 

 side. He describes a glandular, bi-crural ganglion at the base of 

 each of these cysticles, interprets the cysticles as " eyes," and the two 

 crura of the ganglion as optic nerves. Ehrenberg also describes a 

 smaller ganglion, shaped like the above, between each pair of the 

 marginal tentacles.^ Mr. Huxley is disposed, from the analogy of 

 what he has observed in Rhizostoma and Phacellophora, to regard 

 the so-called "optic nerves" as merely the thickness of two superim- 

 posed membranes, '*' and a very similar explanation may be given of 

 the intertentacular ganglia, which appear to be nothing more than 

 the optical expression of the thickened walls of the circular canal."** 



With regard to the part, which, from its characteristic constancy 



* CXLVII. p. 4. t Prep. No. 55. % CXLY. p. 424. § CXI.H 



II CXLVn. p. 3. ^ CXLI. p. 572. ** CXLV. p. 418. 



