316 CONTRIBUTIONS TO THE NATURAL HISTORY 



tract, and all these innumerable interwoven fringes unfold, contract, and disappear, 

 reduced, as it were, to one little drop of most elastic India-rubber. Week after 

 week. I have preserved these animals alive, and have never been tired of com- 

 paring again and again their changes in these thousandfold developments of their 

 appendages. I have called together those who felt the slightest curiosity for such 

 objects, to witness these phenomena, and have found them all interested to the utmost ; 

 and if I have any thing to regret, it is not the time lost in this contemplation, — for the 

 more I became familiar with the sight, the more I was compelled to admire its beauty, 

 and to contrast with the new forms presenting themselves before my eyes those different 

 states with which I had been familiar before, — but it is the circumstance that the 

 duration of their life is limited to a few months, and that I could not have a larger 

 number of philosophic observers to contemplate with me these marvels, and that 

 the time was too short to trace all the details of their structure microscopically ; 

 although I am aware that I have noticed many particulars which had been un- 

 noticed before. 



The chief difficulty in the comparative study of the different genera of this family 

 arises from the circumstance, that they move permanently in different directions, 

 some having the mouth naturally turned upwards, and others downwards ; and that, 

 from not having perceived this difference, the parts placed in opposite positions have 

 been compared with each other in the different genera, which on that account require 

 a complete revision of their characteristics. 



The type under consideration, for which I retain the name of Pleurobrachia, 

 as the most ancient applied to species of this particular conformation, is one of those 

 which is deprived of peripheric lobes, that is to say, in which the gelatinous body is 

 undivided, and the mouth constantly turned upwards or forwards when in motion ; 

 while the genus Bolina, to which I shall next call attention, is one of those in which 

 one extremity of the sphere is split into two lobes, between which the mouth is sit- 

 uated, and in which this opening is almost constantly turned downwards when the animal 

 is moving, though sometimes, when the animal is at rest, it turns in the opposite direc- 

 tion, opening widely its two lobes. It will be obvious how great mistakes may- arise 

 from comparing two animals constructed upon the same plan, but kept in a reversed 

 position when contrasted. The difficulty of a thorough comparison of all the genera 

 of this family is further increased by the circumstance, that genera without lobes or 

 with slight indentations move naturally in the same position as Bolina ; or in a po- 

 sition the inverse of that of Pleurobrachia. Such is the case, for instance, with Alcinoe, 

 while Dellechiaja, with its very complicated lobes, moves in the position of Pleurobrachia. 



