176 



LECTURE IX. 



which may answer the vague description given, I do not in reality 

 find what has been said to exist in that part of the animal." * 



The most conspicuous, if not the most typical, member of the 



physograde order 

 of Acalephaj is the 

 Physalia {fig^ 79.), 

 in which all that 

 part answering to 

 the disc in the "pul- 

 mograde" order, is 

 expanded into a bag, 

 the major part of 

 which is occupied 

 by an air-bladder, 

 whilst the digestive 

 cavity is subdivided 

 amongst a series of 

 appendages attached 

 to one part of the 

 under surface of the 

 bag. This part con- 

 sists of an outer thin 

 and dense mem- 

 brane, of an inner 

 thicker membrane 

 beset with long cilia, 

 and of an air-bladder, 

 which at one point, a, is attached to the above membranes, where 

 there is a small constricted aperture, at least in the outer membrane. 

 This membrane is developed into a kind of crest along its upper part. 

 It is provided with many fine muscular fibres, and the whole bag 

 contracts into a small irregular mass when punctured and the air 

 evacuated ; but it seems that the Physalia has no power of voluntarily 

 emptying its air-bag. 



The appendages are of three kinds, — urticating, digestive, and 

 (probably) generative. The urticating tentacles are the longest; 

 they are hollow, and are provided with muscular fibres, of which the 

 most conspicuous are longitudinal, and serve to retract them ; they 

 contain many corpuscles of a reniform shape, and are richly provided 

 with thread-cells, whose filaments are of the spiral kind. The gastric 

 appendages are shorter and wider, and are provided with stomata, 



Physalia. 



♦ CXLVIII. p. 348. 



