COELENTEBATA 39 



Although the endoderm arises without reference to a definite 

 pole, it is thus evident that polarity is established, and there 

 is every reason for believing that the animal pole of the egg, 

 that from which the polar bodies are liberated, is the anterior 

 pole of the planula larva and the pole of fixation. 



Some of the near allies of Hydra use their tentacles for 

 creeping movements and move about freely. An expansion 

 of the body in the region of the tentacles converts the body 

 into an umbrella shape, and thus a primitive medusa is pro- 

 duced, which has been called an actinula. The actinula is 

 able to swim, and thus an alternation between a creeping or 

 fixed state and a pelagic state has been found in fresh-water 

 forms nearly allied to Hydra. 



Further information will be found in papers by Brauer 

 (1891, 'Zeitsch. fur wissensch. Zool.' Bd. 3; 1909, ' Zool. 

 Anz.' Bd. 33), Chun (1892, Bronn's ' Klass. und Ord. d. 

 Thierreichs,' Bd. 2), Potts (1906, ' Quart. Jour. Micro. 

 Science,' vol. 50). 



A flattened infusorian, Trichodina pediculus, may some- 

 times be seen creeping over Hydra when it is being examined 

 in the living state. It does not appear to produce any harm 

 to its host. Martin 1 pointed out that in the lochs of Scotland 

 Hydra is liable to attack by the Planarian, Microstomum. 

 Hydra is eaten also by insect larvae. 



Obelia. Hydrozoa are very common in the sea, and shore 

 species may be procured readily for examination in the living 

 state. They are usually colonial forms, and they are usually 

 also protected in a case secreted from the ectoderm, and 

 anchored firmly by root-like outgrowths of the base. These 

 compound hydroids are practically confined to the sea, but 

 the genus Cordylophora is common in rivers and canals of 

 the northern hemisphere. 



Obelia is chosen for particular study, for the genus is 

 world-wide in distribution and the species are common at the 

 shore. Obelia geniculata is practically cosmopolitan in dis- 

 tribution : it extends from tropical to temperate and cold 

 seas of both hemispheres. The hydroid is attached to stones, 



1 1908, Quart. Jour. Micro. Science, vol. 52. 



