258 NEW-YORK FAUNA — MOLLUSCa. 
ORDER VI. TUNICATA. 
Marine animals, of a gelatinous substance, varying in form, furnished with membranous 
tunics (often of a leathery consistence) instead of a calcareous covering ; with two aper- 
tures. Sometimes isolated ; often many are united together into a common mass. No 
distinct head. Mouth, vent and gills within; the latter of various forms, but never 
divided into four leaflets. 
Oxss. The animals of this order, according to Cuvier, form a group under the name of 
Acephala nuda, arranged immediately after the testaceous Acephala. By Lamarck they are 
treated as a distinct class, and arranged between the Radiata and Worms. We follow Cuvier 
in considering them as belonging to the class Mollusca, but place them at the end of that 
class. 'They are not numerous in species, or perhaps it would be more proper to say that 
they have not as yet been very extensively examined. ‘They are sessile or free. Some of 
them live isolated, without any organic connexion with each other ; others are united ina 
common mass, but only in the adult state. 
GENUS ASCIDEA. ‘Linneus. Lamarck. 
Animal ovoid, more or less elongated, sometimes cylindrical, very variable in shape, with its 
covering more or less dense; enlarged or pedunculated at its base, and terminated above 
by two short unequal tubes, with the orifices radiated by tentacular papille. 
Oss. This genus is rich in species, and, contrary to the usual law governing the habitat of 
other mollusca, appears to be more abundant and the species larger in northern latitudes. 
They are usually found grouped together, and sometimes growing upon each other. They 
appear to have no means of defence, unless by ejecting water from their two orifices. They 
furnish nutriment to marine animals, and man feeds on some species. The two orifices cor- 
respond to the two tubes of several bivalves, one serving to admit water, and the other to 
give passage to the feces. One of the most common in our waters is a species closely allied 
to the A. rustica of Linneus. Many of those of the American coast have been described by 
Mr. Lesueur in the third volume of the Journal of the Academy of Natural Sciences. 
