NANOMIA. 



77 



large gradually, and soon take upon themselves a perfect Me- 

 dusa structure (Fig. 110), with the exception of the proboscis, 

 the absence of which is easily understood, when we find that 

 these Medusae serve the purpose of locomotion only, having no 



Fig. 109. 



Fig. 110. 



share in the function of feeding the community, so that a diges- 

 tive apparatus would be quite superfluous for them. In every 

 other respect they are perfect Medusae, attached to the Hydra as 

 the Medusa buds always are when first formed, having the (four) 

 chymiferous tubes, characteristic of all Hydroid Medusae, radi- 

 ating from the centre to the periphery ; two of these tubes 

 are very winding, as may be seen in Fig. 110, while the other 

 pair are straight. The Medusae themselves are heart-shaped 

 in form, depressed at the centre of the upper surface, and 

 bulging on either side into wing-like expansions, where they 

 join the stem. These expansions interlock with one another, 

 crossing nearly at right angles. The Medusae-like buds are the 

 swimming bells ; by their contractions, alternately taking in and 

 throwing out the water, they impel the whole community for- 

 ward, so that it seems rather to move like one animal, than like 

 a combination of individuals. 



Fig. 109. Young Nanomia, older than Fig. 108. 



Fig. 110. Heart-shaped swimming bell of Nanomia ; magnified. 



