NOTES. 451 



like many others, has an ait-bladder at the upper end of the sac. The 

 air-bladder has an opening at the top by means of which, when the 

 creatures are kept in small vessels, the air easily escapes. I am asto- 

 nished to find that Studer does not seem to have particularly remarked 

 this feature — and yet it seemed to offer so obvious a parallel with the 

 fishes living at great depths, and provided with air-bladders 1 In these, 

 as has long been known, the air contained in the bladders is exposed to 

 very considerable pressure ; if this is suddenly removed by the fish being 

 rapidly brought to the surface, the air, previously compressed, expands 

 and distends the belly ; a prick allows the air to escape, the air-bladder 

 collapses, and the fish, restored to its natural size, can swim again. It is 

 evident that the air in the deep-sea Rhizophysze must also be much 

 compressed, but in them the perforation already exists by which it can 

 escape when the animal is raised to the surface, and the expanding air 

 threatens to burst the bladder'. Studer says nothing about this very 

 conspicuous expansion of the bladder, and we may therefore suppose that 

 the Bhizophysae had parted with tlie chief portion of their air before 

 reaching the surface. This filling and emptying of the air-bladder, which 

 must undoubtedly exist in the Ehizophysse to enable them to rise or sink, 

 recalls the hydrostatic vacuoles of the Arcellidse. 



Note 97, page 205. Lymusea and Planorbis are frequently to be seen 

 with the sole — so to speak — of the foot spread out on the upper surface 

 of the water, and thus swimming in an inverted position ; but this 

 swimming is more accurately described as creeping on the under surface 

 of the air, the plane of contact of the air and water. At first we are 

 inclined to imagine that the adhesion of the foot to this surface is strong 

 enough to bear the whole weight of the animal and its shell. This, how- 

 ever, is not the case ; for if the snail is induced to retract its foot so 

 slowly that no air-bubbles are expelled from the branchial orifice during 

 the process, the animal turns over in the water, but it remains floating 

 at the surface, so that at that moment its specific gravity must be less 

 than that of water. 



Note 98, pafje 207. It may, perhaps, surprise many readers to hear of 

 the fins of birds. But most water-birds do in fact use their wings for 

 swimming in the water quite as well as for flying in the air. The wings 

 of ducks, divers, cormorants, &c., are not the less true wings. In the 

 penguins, however, the same limbs, morphologically speaking, have 

 become trae fins, which the creature can use in the water, but can no 

 longer use in the air as wings. In them, although the portions of the 

 skeleton still correspond in all essentials with these of a true wing, 

 there is absolutely no external resemblance to the true wings of other 

 birds. Anyone may convince himself of this in a zoological collection. 



Nate 99, paije 2C(9. This assertion is founded on a careful anatomical 

 investigation of a good many diflrerent species — at least six — of the 

 genns Navicella. Naturally there are visible differences between it and 



