C6 



ZOOLOGY. 



such as the medusae of the Hydro! ds and the Trachy- 

 nemldcB. In the European Nausiihoe 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 

 moath;*, gastro-vascnlar cavity; 

 c, ectoderm ; ot, endodemi.^ 

 After MetschnlkofE. 



Fig. 45. — Scyphistoma of Aurelia 

 Jlavidula, at different ages; magni- 

 fied. — After Agassi;!. 



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. 46.— Strobila of Au- 

 relia Jlavidula. — After 



Fig. 4T. — Ephyra or 

 earliest free condition of 

 Aurelia. — Alter 



The Aurelia jlavidula spawns in late summer, the females 

 being distinguishable by their yellowish ovaries, the male 

 glands being roseate, while the tentacles of the females are 



* Jenaisclie Zeitschrift, 1877, p. 355. 



