(333). The bodies of these strangely-constructed creatures are so extremely transparent, that their presence is discoverable with great difficulty even in small quantities of sea-water. They are generally met with at a great distance from land, abounding more especially in the seas of tropical climates. They swim with great facility, their anterior or nuclear extremity being directed foremost; while the water taken into their bodies, being forcibly ejected, by the contractions of their subcartilaginous parietes, through the wide apertures opening backwards, propels them through their native element.

(334). Whilst exercising this mode of locomotion, the long slender filament above alluded to is extended behind, being partially lodged in a groove excavated in the posterior division of the natatoiy organ. It varies considerably in length, being highly contractile, so much so, indeed, that it is sometimes completely withdrawn into the body; and its structure is further remarkable from the circumstance that throughout its whole length it is furnished at regular intervals with minute suckers*. But the true nature of this organ is very imperfectly known; most probably it will be found to be analogous in its real character to the proligerous apparatus of the Salpce, to be described hereafter; indeed, such is the evident relationship between the Diphyea and the Salpoid Tunicata, that it is very doubtful whether they ought not to be classed as members of that group.