The Structure and Habits of Physalia. 363 



inflexion of the pneumatophore or outer sac takes place. This 

 point corresponds with the apex of the more elongated ex- 

 tremity of the pneumatophore. It will thus be seen that there 

 is no communication between the cavity of the pneumatocyst 

 and the general cavity of the organism ; but, on the other hand, 

 that a free communication does exist between the general, or 

 " somatic" cavity as it is called, and the space existing be- 

 tween the walls of the pneumatocyst and pneumatophore. 



According to Eichwald* and Von 01fers,f the crest is 

 formed of a series of vertical cascal folds of the pneumatocyst, 

 invested exteriorly, in common with the rest of that body, by 

 the two membranes of the pneumatophore. It is extremely 

 doubtful whether the Physalidge possess the power of expelling 

 the air from the air-chamber as asserted by some writers. The 

 only reliable evidence of this process is adduced by Esch- 

 scholtz, J who describes having seen the air voluntarily expelled 

 from a young specimen of Physalia only five lines in length, so 

 that it immediately sank to the bottom of the glass vessel in 

 which it was placed for observation. In the nearly-allied families 

 of Ehizophoridge and Physophoridse, in which the pneumatocyst 

 consists only of a minute spherical or pyriform vesicle, I have 

 repeatedly seen the creatures suddenly sink to the bottom of 

 the glass on being irritated, but without any appearance of 

 collapse of the air-cell. In the young Physalidas the character 

 of the air-cell (" pneumatocyst") is identical with that of the 

 adult Physophora and Rhizophora, but no amount of irritation 

 ever caused the specimens to contract the air-cell or sink, 

 although their polypites and tentacles exhibited sensibility to 

 the slightest touch, or even vibration of the glass, by becom- 

 ing instantly coiled up close to the body. 



During the long- continued calms at the equator, extending 

 sometimes over several days, when the surface of the sea is 

 literally as smooth as a mirror and as pellucid as crystal, I 

 have had ample opportunity of watching Physalia, and have 

 in no instance observed the float collapse, or the creature sink 

 beneath the surface. Under the above conditions it becomes 

 manifest that the creature is wholly devoid of power to move 

 to and fro ; the individuals remaining, as it were, fixed in the 

 same spot, and evincing no signs of vitality beyond a partial 

 collapse of the crest, occasional abrupt changes in the direction 

 of the axis of the pneumatophore after the fashion recorded by 

 M. de Quatrefages§ and Professor Huxley (loc. cit), a gentle dip 

 over on one side — probably with a view to moisten the surface 



* Mem. de I'Acad. Imp. des Sciences de St. Petershourg, 1824. 



f Abhandlungen de Kon. Akad. de WissenscJiaften zu Berlin, 1831. 



J System du Acalephen, 1829. 



§ Annates des Sciences Naturelles, 1853. 



