66 



ZOOLOGY. 



such as the medusas of the Hydro! ds and the Tracliy- 

 nemidce. In the European NausitJioe albida and Pelagia 

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

 owing to their deeply indented disks. There are instead eight 



Fig. 44.- Gastrula of an Anre- 

 Ha-like Medusa, a. primitive 

 mouth; ft. gastro-vascular cavity; 

 c, ectoderm ; d, endodeim. 

 After Metschuikoff . 



Fig. 45. Scyphistoma of Aurelia 

 f iiriti >ila, at different ages; uiagni- 

 lied. After Agassiz. 



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 aurita 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.4fi. Strobilaof.4- 

 relia flavidula. After 

 Agassiz. 



Fig. 47. Ephyra or 

 earliest frc<; condition of 

 Aurelia. After Agas- 

 siz. 



'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. 



* Jenaische Zeitschrift, 1877, p. 355. 



