NERVOUS SYSTEM OF JELLY-FISHES. 65 



nettles, sun-fish or Acalephs, of which there are about 

 ninety known species on the eastern coast of the United 

 States, we may study as the type of the suborder the 

 common Aurelia Jlavidula Peron and Lesneur of our 

 coast, which is closely allied to the Aurelia aurita of the 

 European shores. It grows to the diameter of from eight 

 to ten inches, becoming fully mature in August, the young 

 appearing late in April in Massachusetts Bay, being then 

 not quite an inch in diameter. The mature ones may be 

 easily captured from a boat or from wharves. On a super- 

 ficial examination, as well as by cutting the animal in halves 

 and making several transverse sections with a knife, the lead- 

 ing points in its structure may be ascertained. Its tough, 

 jelly-like disk is moderately convex and evenly curved, while 

 four thick tentacles depend from between the four large geni- 

 tal pouches ; the tentacles unite below, forming a square 

 mouth-opening, the edge of Avhich is minutely fringed to the 

 end of the tentacles. On the fringed margin are eight eyes, 

 each covered by a lobule and situated in a peduncle, and 

 occupying as many slight indentations, dividing the disk 

 into eight slightly marked lobes. The subdivisions of the 

 water-vascular canals or tubes are very numerous and anas- 

 tomose at the margin of the disk, one of them being in 

 direct communication with each eye-j)eduncle. When in 

 motion the disk contracts and expands rhythmically, on the 

 average twelve or fifteen times a minute ; on the approach 

 of danger they sink below the surface. 



"While a distinct nervous system has not been discovered 

 in Aurelia, Romanes suggests that there are primitive nervo- 

 museular cells, such as those shown by Kleinenberg to exist 

 in Hydra, and he concludes, after a series of experiments 

 on Aiirelia aurita, that the whole contractile sheet of the 

 bell presents not merely the protoplasmic qualities of ex- 

 citability and contractility, but also the essentially nervous 

 quality of conducting stimuli to a distance irrespective of 

 the passage of a contractile wave. The later researches of 

 0. and E. Hertwig show that the nervous system of 

 Acalephae (Acraspedota or covered-eyed Medusa) is much 

 more primitive than in the naked-eyed or craspedote forms, 



