66 



ZOOLOGY. 



such as the medusfe of the Ilydroids and the Trachy- 

 neniidce.. In the European JSfausitJioe albida and Pelagia 

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

 owing to their deeply indented disks. There are instead eight 



Fiff. 44. — Gastrula of an Aure- 

 lia-Iike Afedusa. a, primitive 

 month ; b, gJlstro-vasculav cavity; 

 c. ectoderin ; d, ejidoderm. — 

 After MetschniltofE. 



Fig. 45. — Scyphistoma of Anrelia 

 fiaviiluia^ at different ages; magni- 

 lied. — After Agaseiz. 



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. 

 Avrelia 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. 40.— Sta-otiila of Au- 

 rflia fiavldula. — After 

 Agassiz. 



Fig. 47. — Ephyra or 

 earliest free condition of 

 Avrelia. — After Agas- 

 sis. 



Tbe 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 



Jenaiscbe Zeitscbrift, 1877, p. 355. 



