cause the Gastrula arises from and passes through stages which exist as independent, permanent organisms among the Protozoa.
Needless to say this Gastræa-theory has been violently attacked in detail, with the result that various modifications of the Gastrula, until then undreamed of, have become known.
'Monographie der Medusen.' Jena, 1879-81. With 72 coloured plates.
'Reports on the Scientific Results of the Voyage of H.M.S. Challenger.' With 230 plates:
1. Deep-sea Medusæ. 1881.
2. Radiolaria. 1887.
3. Siphonophoræ. 1888.
4. Deep-sea Keratosa. 1889.
A short holiday spent on the coasts of the Red Sea produced the volume 'Arabische Korallen' (Berlin, 1876); and a longer trip to Ceylon has been described in 'Indische Reisebriefe,' of which the third edition
