MOLLUSCA—TUNICATA. 771 
CLASS IIJ.—TUNICATA. 
Gelatinous, or coriaceous, biforous, bitunicated animals, isolated, in groups, or 
often joined together in a common mass. 
Tue place which the animals of this class ought to occupy in the arrange- 
ment corresponding to their organization, has not been satisfactorily ascer- 
tained. Cuvier places them among his molluscous animals, in the class 
Acephala, and makes them the second order of this class, under the title or 
acephalous animals, without shells; while Lamarck arranges them between 
the Echinodermata and worms. Latreille places them after the Entozoa, 
and they form the fourth order of Blainville’s class, Acephalophora, under 
the name of Heterobranchiata. In point of fact, there seems to be, both 
among the vertebral and invertebral animals, more than one series of forms 
and structure, which, either in the descending or ascending scale, where the 
most nearly-allied groups, in point of structure, are arranged in sequence, 
will always interfere to disturb any continuous or subordinate arrangement. 
The existence of these parallel groups presents formidable difficulties to the 
classification of animals in one unbroken series; but the establishment of 
closely connected groups into natural families,a plan which has been largely 
adopted, by the recent writers on the classification of animals, renders the 
arbitrary limitations of systematic writers, of objects in themselves unlimit- 
ed, a matter of less consequence. We have, therefore, followed M. Cuvier 
in placing the class Twnicata, under the general head Mollusca. 
The animals of the class T'unicata have an oblong, irregular body, as if divid- 
ed interiorly into many cavities. They have no head; possess no distinct 
organs of sensation; and no symmetrical or similar parts in pairs. Some 
tubercles and threads, discovered in their body, are presumed to form the 
aervous system. The body is besides composed of muscular fibres, and dis- 
linct blood vessels; the alimentary tube is open at both ends, and a mass of 
gemme or ova, either solitary or together, in a common envelope, seem to 
form the ovaries. The respiratory organ in this class is always interior, 
formed of two membranous, reticular leaflets, sometimes constituting a 
sort of sac, sometimes forming two bands of unequal length, united by one 
end. None of these animals possess a retractile tube for locomotion. Their 
body soft, or coriaceous, is generally fixed either by itself, or in connection 
with others of the species, to foreign substances. No trace of sexual organs 
has been discovered. Many of the animals of this class, from their union in 
a common mass, seem at first sight to form compound animals, like the 
polypi; but this wide distinction is to be remarked between them and the lower 
families, in the zoological scale, that the aggregated Tunicata are independen 
and individual beings, each being provided with a mouth and an aperture for 
