TUNICATA.
This class was first instituted by Lamarck in the year 1816, and has now been generally adopted by naturalists. It includes the Acephales sans coquilles" of the system of M. Cuvier, and embraces several of the zoophytes as well as mollusca of the system of Linnxus.
The animals or this class are all naked, the external covering being soft and coriaceous. There are two apertures, one for the use of the gills in respiration, and the other for the digestive organs. These apertures are frequently surrounded with productions in the form of tentacula. The inner cloak is in many genera loose, in others adhering, but in all it is united with the external one at the two apertures. These sacs are furnished with muscular bands and filaments ; and traces of a ner vous system have been perceived. The alimentary canal is very simple, and scarcely can be distinguished into gullet, stomach, and intestine. The liver adheres to the stomach, and in many is divided into distinct lobes. The gills cover the walls of the cavity of the inner tunic. They are in the form of ridges more or less compli cated. The circulating system appears to be reduced to a single systemic ventricle. The organs of reproduc tion consist of an ovarium, either simple or complicated, with some additional glands, whose uses are not ascer tained.
All the animals of this class live in the sea, and are very widely distributed. Some of them are fixed to rocks and sea-weeds, others move about in the water. Many genera possess species whose individuals are detached and independent, while with others there is an insepa rable union. These united individuals constitute a sym metrical mass, in some cases capable of moving about in the surrounding element.
The investigation of the structure of this class of animals has been conducted with great care and suc cess by M. Savigny, in his " Recherches anatomiques sur les Ascidies composees et sur les ,lscidies simples," inserted in his " illenzoires sur les Animans sans Ver tares." Paris, 1816. MM. Desmaret and Lesueur have likewise contributed materially to unfold their structure. The labours of these authors, together with Cuvier's papers on the genera Salpa and Ascidia, em brace nearly all the information which has been obtain ed regarding the structure and physiology of the animals which have been brought together in this class. It is our intention to give a brief exposition of the systemati cal characters of genera, according to the method fol lowed by M. Savigny.