66 



/.OOLOGY. 



sucli as the medusae of the Hydroids and the Tracliy- 

 nemidce. In tlie European Nansithoe albida and Pelugia 

 noctiluca no nerve-ring is present, for this is impossible 

 owing to their deeply indented disks. There are instead eight 



Fig. 44. Gastrulaof an Anre- 

 lia-like Medusa, a. primitive 

 mouth: b, gastro-vascular cavity; 

 c. ectoderm ; <l. cmloderm. 

 After Metf-chuikoff. 



Fig. 45. Scyj)histoma of Aurelia 

 flavidula, at different ages; magui- 

 lied. After Agassis. 



separate nerve-tracts which unite with the sense-organs in a 

 special elevation of the edge of the disk, forming so-called 

 sense-bearers, which alternate with the eight tentacles. 

 Aurelia aurila has a similar disconnected nerve system.* 



Eimer confirms these discoveries, and states that the ner 

 vous system in these Hydrozoa arises from the ectoderm. 



Fig. 46. StroMla of Au- 

 rflia fiaviduki. After 

 Agasaiz. 



Fig. 47. Kphyra or 

 carliol t'i'i'f condition of 

 Aurelia. After Agas- 

 Biz. 



The Aurelia flavidula spawns in late summer, the females 

 being distinguishable by their yellowish ovaries, the male 

 glands being roseate, while the tentacles of the females are 



* Ji-naische Zeitschrift, 1877, p. 355. 



