CG 



ZOOLOGY. 



such as the medusae of the Hydroids and the Tracliy- 

 nemidiu. In the European Nansithoe 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- 

 lia-like Medusa, a. primitive 

 mouth ; b, gastrp-vascnlar cavity; 

 c, ectoderm ; d, eudoderni. 

 After Metschnikoff. 



Fig. 45. Scyphistoma of Aurelia 

 Jlavidula, at different ages; magni- 

 fied. 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 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. 4(5. Strobila of Au- 

 rr-lia flavidula. After 

 Agassiz. 



iir. 47. Ephyra or 

 c-l free 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. 



