(1259). The singular class of Mollusca to which the name at the head of this chapter has been applied is at once distinguished by the remarkable character afforded by the texture of the external investment of the body. In their general organization the Tunicata are very nearly allied to the ordinary inhabitants of bivalve shells, with which, both in the structure and arrangement of their viscera, they correspond in many particulars; but instead of being enclosed in any calcareous covering, a strong, flexible, cartilaginous or coriaceous integument forms a kind of bag encasing their entire body, and only presenting two comparatively narrow orifices, through which a communication with the exterior is maintained.

(1260). Various are the forms under which these animals present themselves to the eye of the naturalist; but the enumeration of them will be more conveniently entered upon hereafter. We shall therefore at once lay before the reader the principal points connected with the structure and habits of an Ascidia belonging to one of the most perfectly organized families; and after examining this attentively, our descriptions of allied genera will be rendered more simple and intelligible. The Ascidians are abundantly met with upon the shores of the ocean, especially at certain seasons of the year. In their natural condition they are found fixed to the surfaces of rocks, sea-weed, or other submarine bodies: frequently, indeed, they are glued together in bunches; but in this case individuals are simply agglomerated, without organic union. Incapable of locomotion, and deprived of any external organs of sense, few animals seem more helpless or apathetic than these apparently shapeless beings, and the anatomist is surprised to find how remarkably the beauty and delicacy of their interior contrasts with their rude external appearance.

In the species selected for special description (Phallusia nigra), the external envelope (fig. 243, a a a) is soft and gelatinous in its texture, fixed at its base to a piece of coral (I), and exhibiting at its opposite extremity two orifies (h, f), placed upon prominent portions of the body. Through the most elevated of these orifices (h) the water required for respiration and the materials used as food are taken in, while the other (f) gives egress to the ova and excrementitious matter. The soft outer covering is permeated by bloodvessels, which ramify extensively in it; it is moreover covered externally with an epidermic layer, and lined within by a serous vascular membrane, which, in the neighbourhood of the two orifices, is reflected from it on the body of the animal lodged inside. The creature hangs loosely in its outer covering, to which it is only connected at the two apertures by means of the reflexion of the peritoneal membrane above mentioned.

* Tunicatus, clad in a tunic.

(1261). On removing a portion of the exterior tunic, that in reality represents the shells of a bivalve Mol-lusk, the soft parts of the Ascidian are displayed. The body is seen to be covered with a muscular investment (the mantle) (fig. 243, b b, c), composed of longitudinal, circular, and oblique fibres, which cross each other in various directions, so as to compress by their contraction the viscera contained within, and this so forcibly that, when alarmed, the animal can expel the water from its branchial sac, immediately to be described, in a thin continuous stream, sometimes projected to a distance of many inches.

(1262). Respiration is effected in an apparatus of very peculiar contrivance, to the examination of which we must now request the attention of the student. A considerable portion of the interior of the body is occupied by a circumscribed cavity, that opens externally by the orifice h; into this bag a bristle has been introduced in the dissection, represented in the figure (fig. 243); its walls are seen to be composed of a thin but very vascular membrane (d d d), that has been partially turned back, so as to display the interior of the respiratory sac. The membrane (fig. 243, d d d; fig. 244, e), when examined with a microscope, is found to be covered with a magnificent network of blood-vessels, formed by innumerable canals uniting with each other at right angles; and moreover, when seen in a living state, its surface is discovered to be densely studded with vibratile cilia, whose rapid action constantly diffuses fresh supplies of water over the whole vascular membrane. The respiratory cavity has but one orifice for the admission of water (fig. 244, a), and this is guarded by a fringe of delicate and highly sensitive tentacula (fig. 244, b); so that the water, as it is drawn into the body, having necessarily to pass these tactile organs, any foreign substances which it might contain of a prejudicial character are at once detected and denied admission. All the vascular ramifications spread over the lining membrane of the branchial chamber are connected with two sets of large vessels; one of which, receiving the blood from the body, disperses it over the spacious respiratory surface; while the other, collecting it after it has undergone exposure to the respired medium, conveys it in a pure state to the heart.

Structure of Phallusia nigra: a a a, external envelope; b b, the mantle.

Fig. 243. Structure of Phallusia nigra: a a a, external envelope; b b, the mantle; c, mantle reflected so as to display d d, the membrane lining the respiratory sac; e e, alimentary canal; f, excretory orifice; g, orifice of oviduct; h, oral aperture; l, a piece of coral to which the animal is fixed; n, the anus. (After Hunter).

(1263). The heart itself presents the simplest possible form, being generally a delicate elongated contractile tube, receiving at one extremity the blood derived from the numerous vessels that ramify over the interior of the branchial sac, whilst at the opposite end it becomes gradually attenuated into the aorta, through which it impels the circulating fluid and disperses it through the system.