In the third period of development the embryo takes on the ascidian character. The development of the internal organs progresses, and the external orifices become apparent (fig. 781. u). At the commencement of this period, the form of the young Ascidian varies considerably. The numerous appendages dis appear, just as the caudal appendage was lost. The embryo becomes rounded and larger, and its substance is distinctly seen to be disposed in three layers. Up to the present time the vitelline cavity has had no external communi cation, but the mucous cavity is now extended on one side, to form the mouth. The mucous layer is elongated also on the opposite side, but instead of reaching the exterior, it is folded back on itself, forming the intestinal loop. The yolk soon opens a passage on each side, constituting the oral or branchial, and the ventral or anal, orifices. The intestinal tube becomes completed by its separation into the respiratory, the digestive, and the cloacal cavities. Around each external orifice certain nipple-like, palpiform bodies appear, which soon lengthen, but before long they are hidden by the integument (fig. 781. t). The eyelike pigmentary spot still continues for some little time, and is situated in the middle of a band (the oesophageal collar) that seems to embrace the respiratory cavity. At the
base of the intestinal cavity, the opaque concretionary body appears, to which the heart is subsequently attached, and which Van Beneden considers to be analogous to the internal shell of the Limaces. This is furnished with vibratile cilia, which occasion circulatory currents, until the heart makes its appearance in the form of a thin, gently pulsating, mem branous body. Contemporaneously with these cilia, others appear in the interior of the bran chial sac, in the form of a ciliated„circlet, to which a second and afterwards others are added, forming altogether the peculiar bran chial tissue. These cilia give rise to currents in the digestive and subcutaneous cavities. The young Ascidian can now carry on the functions of respiration and digestion.
At the surface of the skin, and around the anal and branchial cavities there are apparent longitudinal stria., which are evidently the rudi mentary muscular bands. The sphincters, also, at the two extremities of the intestinal tube, are brought into action. Some slight further modifications only are now required to perfect the development of the little Ascidia.