MOLLUSCA — TUNICATA. 771 



CLASS III. — TUNIC AT A. 



Gelatinous, or coriaceous, biforous, bitunicatcd animals, isolated, in groups, or 

 often joined together in a common mass. 



The 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 of 

 acephalous animals, without shells ; while Lamarck arranges them between 

 the Echinodermata and leorms. Latreille places them after the Entozoa, 

 and they form the fourth order of Blainville's class, Acephalophora, under 

 the name of Heferobranchiata. 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 Tunicata, under the general head MoUusca. 



The animals of the class Tumcata 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 

 nervous system. The body is besides composed of muscular fibres, and dis- 

 tinct blood vessels ; the alimentary tube is open at both ends, and a mass of 

 gemmae 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 independent 

 and individual beings, each being provided with a mouth and an aperture for 



