In the first section, the animals are furnished with a disk or umbrella of varying shape, which serves as a float, beneath which hang certain processes connected with the functions of prehension and digestion.  In this division are included some of the best-known forms.  The creature, in this case, propels itself by the alternate contraction and expansion of its disk, thus striking the water, and driving itself forward.  These movements take place at regular intervals, and serve a double purpose.  They not only propel, but at the same time drive the water over the lower surface of the disk.  Here is situated a complicated net-work of vessels, and the fluids of the body are thus exposed to the influence of oxygen, and receive the needed aeration.  The stroke of the disk, therefore, is not only a locomotive, but also a respiratory act.  The jelly-fishes of this section move as they breathe, and breathe as they move.  Hence the name which has been given them—­Pulmonigrades.  We find the same admirable economy of resources amongst the lower animalcules.  The cilia which propel them secure the aeration of the system.

It is evident that the motive apparatus in this section of the Acalephae is but a feeble one.  It only avails in calm weather.  When the sea is agitated, the jelly-fish is driven helplessly along.  It cannot choose its path.  As its food, however, is everywhere abundant around it, and it has no business that should lead it in one direction more than another, there is no great hardship in this.

In this section are included some of the most beautiful, as well as common of the tribe.  The forms of the umbrella are often most lovely, and present an astonishing variety.  As an example of the beauty which they sometimes display, we may refer to a species which resembles an exquisitely formed glass-shade, ornamented with a waved and tinted fringe.  The most perfect grace of form, the transparency of the crystal, and colour as delicate as that of the flower, combine to render this frail being one of the loveliest of living things.

In another section, locomotion is effected by a modification of ciliary apparatus.  We have a familiar example in the Beroe of our own seas, a most attractive little being, and a prime favourite with naturalists, who have described its habits and celebrated its beauty with enthusiasm.  We shall not soon forget the delight with which we first made acquaintance with this graceful little rover.  While rambling along the shore in quest of marine animals, our attention was arrested by a drop of the clearest jelly, as it seemed to be, lying on a mass of rock, from which the tide had but just receded.  On transferring it to a phial of sea-water, its true nature was at once revealed to us.  A globular body floated gracefully in the vessel, scarcely less transparent than the fluid which filled it.  Presently it began to move up and down within its prison-house, and the paddles by means of which the beroe dances