MEDUSA OE MICEOHYDKA RYDEBI. 639 



In the case of Microhydra tlie Hydroid appears to attach 

 itself to rocks and stones in swift-running streams. 



Under such conditions long flexible tentacles, like those 

 possessed by the common freshwater Hydra, would stream 

 out with the current, and be of little use for the catching of 

 food. One would rather expect to find in such a situation a 

 Hydroid with very short and fairly stiif tentacles, like those of 

 Cory ne, which lives between tide-marks. But Microhydra 

 has, perhaps, conquered its new habitat at the expense of 

 its tentacles, as it may be reasonably assumed that this 

 Hydroid is descended from one which formerly inhabited 

 the sea. 



There is, however, a difference between the Hydroid phase 

 of Lininocodium and that of Microhydra. The Hydroid of 

 Limnoc odium secretes from its body a glutinous mucus, to 

 which adhere particles of mud and other debris, so that a 

 protecting case is formed round the body, leaving only the 

 oral end free, and this end is capable of contracting within 

 the tube. 



The Hydroid of Microhydra, so far as I can judge from 

 the figures and description, forms no protecting case to its 

 body. Potts' figure 25 shows the Hydroid attached to the 

 glass of the aquarium with the ''base and adjacent parts 

 showing adherent threads of Nostoc and adventitious 

 particles." Any one who has kept a freshwater aquarium 

 knows that the glass becomes thickly coated with unicellular 

 Algae. Mr. Potts figures this coating surrounding the base of 

 the Hydroid, the body of the Hydroid being shown by him 

 quite naked. 



Medusa. — The comparison between the Medusa of Limn o- 

 codium and of Microhydra is not so simple as that of their 

 Hydroids. In the first place a stage exactly similar to that 

 of Microhydra has not been described and figured for 

 Limnocodium. 



Fowler has described the Medusa-bud on the Hydroid of 

 Limnocodium at a very early stage, whilst still attached to the 

 polyp (PI. 35, fig. 1), but as his supply of material failed he was 



