192 



THE MECHANICAL FUNCTIONS. 



§ 5. Acalepha. 



Floating masses of living gelatinoHS matter 

 are met with in every part of the ocean ; often 

 in vast numbers, and of various forms ; and hav- 

 ing but little the appearance of belonging to the 

 animal kingdom. They compose the order Aca- 

 lepha, of which the 

 Medusa (Fig. 81) may 

 be taken as the type. 

 They appear, from 

 their organization, to 

 be raised but a single 

 step above polypi ; and 

 in point of activity and 

 locomotive powers, they 

 rank among the lowest 

 of those Zoophytes 

 which are not permanently fixed to the spot 

 wliere they were first developed. They are 

 almost wholly passive beings, floating on the 

 surface of the sea, or remaining at a small depth 

 below it, carried to and fro by the motion of 

 every tide and current, and destined to be the 

 rniresisting prey of innumerable tribes of animals 

 which people every part of the ocean. 



The usual form of a Medusa is that of a hemis- 

 phere, with a marginal membrane, like the fold 



