_a._ Upper lip or posterior orifice. _b._ Anterior orifice. _c._ Prolongations of the test by which the animal is adherent to its neighbours. ] [Illustration: Salpæ, isolated and associated. A. _Salpa runcinata_, solitary. B. _Salpa runcinata_, associated. C. _Salpa zonaria_, aggregated. ] While the sessile Ascidiæ remind one of the polyps, the transparent Salpæ, freely swimming in the sea, bear a great resemblance to the pellucid jelly-fishes. Each resembles a crystalline tube, through which one can distinctly see the internal coloured parts. Sometimes these animals, which abound in the warmer seas, are found solitary, at other times associated in circular or lengthened groups, termed garlands, ribands, and chains; but, strange to say, these two forms so different in outward appearance are only the alternating generations of one and the same animal. The chained Salpæ produce only solitary ones, and the latter only chains, or, as Chamisso, to whom we owe the discovery of this interesting fact, expresses himself, "a salpa mother never resembles her daughter, or her own mother, but is always like her sister, her grand-daughter, or her grand-mother." When Chamisso first made known his discovery, he was laughed at as a fanciful visionary, but all later observations have not only fully confirmed his statement but also discovered similar or even more wonderful metamorphoses among the jelly-fish, polyps, crustacea, sea-urchins, and other marine animals. Thus Chamisso gave the first impulse to a whole series of highly interesting observations, and his rank is now as well established among naturalists as it has long been among the most distinguished poets of Germany. The Salpæ progress by the alternate contractions and dilatations of their tubular body. In this manner the chains, as if obeying a common impulse, glide along with a serpentine movement, and are often regarded by sailors as sea-snakes. [Illustration: Inner or under side of the superior plated surface of Chelyosoma Macleayanum. _a._ Branchial orifice. _b._ Anal orifice. _c._ Muscles bordering the carapace-plates. _d._ Central hexagonal plate. _e._ Surrounding plates. _f._ The nerve-ganglion and nerve-fibres. _g_, _h_. Auditory apparatus. _i._ Row of tentacles, anterior to the œsophagus. _j._ Stomach. _k._ Part of the intestine. ]