The animals we have hitherto been considering were all regarded by Cuvier as belonging to his first class, the Zoophytes, and are continued therein by Carus; the latter, however, allows that the Echinoderms are somewhat removed from the class by the commencement of a nervous system. Lamarck's next Class, the Tunicaries, which we are now to enter upon, form part of the headless Molluscans of Cuvier, and belong to that section of them that have no shells. My learned friend, Savigny, in his elaborate and admirable work on The Invertebrate Animals, who also considers them as a separate class, denominates them Ascidians, dividing them into two Orders, Tethydans and Thalidans. Many alcyons of Linné and others, are now referred to the Class we are treating of.

The characters of the class may be thus stated: Animal, either gelatinous or leathery, covered by a double tunic, or envelope. The external one, analogous to the shell of Molluscans, distinctly organized, provided with two apertures, the one oral, for respiration and nutrition, the other anal; the interior envelope, analogous to their mantle, provided also with two apertures adhering to those of the outer one. Body oblong, irregular, divided interiorly into many cavities, without a head; gills occupying, entirely or in part, the surface of a cavity within the mantle; mouth placed towards the bottom of the respiratory cavity between the gills; alimentary tube, open at both ends; a ganglion, sending nerves to the mouth and anus.