SENSES OF THE MEDUSAE. Ill 



of the ramifying alimentary tubes, which has been mistaken for a 

 circulation, but which is merely the effect of ciliary action, or of peri- 

 staltic movement in the walls of the intestine. 



(285.) Up to the period when Ehrenberg made the important re- 

 searches we are laying before the reader, relative to the anatomy of 

 these creatures, it was impossible to account for the capability of loco- 

 motion which the Pulmonigrade Acalephse evidently possess, but which 

 his researches serve to render perfectly intelligible. The canals 

 formed by the ramifications of the alimentary apparatus he observed 

 to be all bordered by two delicate lines of a pale red colour, which, 

 under the microscope, are evidently of a muscular character; by the 

 contractions of these, therefore, the most important movements of the 

 animal are accomplished. Besides the above, however, other muscles 

 are discernible. In Cyanea the disk is surrounded with a fringe of 

 tentacula, each of which exhibits at its base a muscular structure ; 

 consequently the possession of muscular fibre is evidently established 

 as a part of the economy of these animals. 



(286.) It is very probable that the older writers, who speak of a 

 circulation of blood in the Medusae, only alluded to the movements ob- 

 servable in the contents of the intestinal ramifications; it appears, 

 however, from Ehrenberg's observations, that in the MedusaB there 

 exist distinct globules, which are of a uniform round shape, enclosed in 

 distinct vessels, wherein a kind of circulation is carried on : these glo- 

 bules he describes as being colourless, spherical, simple, and varying 



from yirg-tk ^ lTFo^ n ^ a ^ ne *- diameter. 



(287.) Although the Medusae have always been admitted to possess 

 considerable sensibility, no traces of a nervous system had been de- 

 tected in their soft and delicate tissues until Ehrenberg pointed out a 

 structure apparently of a nervous character. On carefully examining 

 the eight brown-coloured spots which are disposed at equal distances 

 around the margin of the disk (fig. 48, F, /), he found them to present a 

 very elaborate and remarkable organization. Each of these coloured 

 spots is seen, when accurately observed, to be composed of a little 

 button-like appendage, of an oval or cylindrical shape, attached to the 

 extremity of a slender pedicle, which in turn takes its origin from a 

 kind of vesicle, wherein may be perceived, by means of the microscope, 

 a glandular-looking substance. On the dorsal aspect of each of the 

 pedunculated brown -coloured appendages is situated a distinctly marked 

 round spot of a bright red colour, supposed by Ehrenberg to be an 

 ocular organ, while he considers the glandular-looking substance above 

 mentioned to constitute a nervous ganglion. In addition to this ar- 

 rangement, he considers that there exist, running all along the margin 

 of the disk, in each of the interspaces between the marginal tentacles, 

 a series of ganglia of a similar character, giving off nerves to the ten- 

 tacula, whilst other ganglia are to be detected in the tentacular append- 



