Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 92 TUNICATA. A tail, which by its undulating movements propels the body forward. In order to understand the structure of the Ascidians, it will be well to start from these simply organised forms. The most striking character of the Appendicularia, next to the possession of the ventrally-placed swimming tail with its notochord-like skeletal axis (urochord), consists in the absence of a cloacal chamber for the reception of the excreta. The anus is placed in the middle line of the ventral su
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 92 TUNICATA. A tail, which by its undulating movements propels the body forward. In order to understand the structure of the Ascidians, it will be well to start from these simply organised forms. The most striking character of the Appendicularia, next to the possession of the ventrally-placed swimming tail with its notochord-like skeletal axis (urochord), consists in the absence of a cloacal chamber for the reception of the excreta. The anus is placed in the middle line of the ventral surface ; further, there are two funnel-shaped atrial canals which begin on either side with a strongly-ciliated opening into the pharyngeal sac, and open to the exterior right and left, visually rather in front of the anus. These branchial passages arise as invaginations of the ecto- derm, which come into connection with corresponding evaginations of the pharyngeal sac. The in- troduction of nourishment is regulated by two ciliated arcs, which begin at the front end of a short endostyle, surround the entrance of the pharyngeal sac, and run obliquely towards the dorsal surface, where they unite to form a median row of cilia (composed of two rows of ciliated Cells). The latter passes back fco the opening of the O3SOphagUS, , .,. , opposite a narrow ventral ciliated band, which begins at the hind end of the endostyle (fig. 562). The Ascidian larvae (PhaUusia) also have, as Krohn long ago discovered, two branchial slits with corresponding atrial passages. The latter, according to Kowalevski, arise as invaginations of the ectoderm, and later on unite on the dorsal side, and then open by a common cloacal orifice. The ectodermal lining of the atrial cavity, which grows round the sides of the pharyngeal sac, consists, therefore, of a branchial layer which is applied to the pharynx, and a parietal layer which forms the internal lining of the outer wall of the peri- branchial or atrial ca
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