The _Holothuriadæ_, or Sea-Cucumbers, are very singular creatures; their form, as it floats in the waters, exhibiting as good a miniature representation of a small pickled Gherkin as can be conceived, except in colour, the shells or cases of these animals being generally white. One of the species, _Hyalina_, has a case which seems formed of crisp rice-paper, and is covered with spines of the same colour and texture. The tentacula, or breathing apparatus, eight in number, are curiously branched, and, when expanded, have the appearance of a skeleton flower, of which the figure in Plate IX. will convey a tolerable idea. The functions of this flower-like set of organs are probably the same as those of the _Nudibranch_ class of _Molluscs_, which, though generally considered as being a breathing apparatus, are, probably, at the same time food-collecting organs, as all the creatures thus furnished are liquid feeders. _Thyone papillosa_, one species of Sea-Cucumber, has ten branches to this set of organs, which it seldom displays when in captivity; but a little gentle motion artificially imparted to the water, as suggested in another place, would probably produce the kind of excitement requisite for their expansion, as the introduction of fresh water to the tank seldom fails to produce this effect for a time. When irritated, these creatures have the capacity of committing self-destruction in a most determined and complete manner, by expectorating the whole of the intestines and leaving their case or shell bare and empty. But Sir J. Dalzell has observed that the shell thus deprived of its living inmate, must be much more intimately connected with its life and organization than the shells of the Molluscs; for after a considerable lapse of time, he observed that the rejected parts have been renewed by gradual growth. The introduction of a single drop of fresh water will at once drive the creature to this summary mode of putting an end to the inconvenience. OF THE NAKED OR SHELL-LESS MOLLUSCS. The Sea-Lemon, _Doris tuberculata_, is one of the most attractive. It derives its popular name from its peculiar form, which is like that of half a Lemon cut longitudinally. It is generally of a yellow tone of colour also, which greatly adds to the fancied resemblance. It has its breathing apparatus exposed externally, like other Nudibranchs, spreading over the mantle, near the head, in a flower-like shape; and, as it moves slowly round and round the Aquarium, forms a very singular object.