. Elements of Comparative Anatomy. ALIMENTARY CANAL OF TUNICATA. 399 Respiratory Chamber (Brancliial Sac). § 309. The simplest condition—that found in the Copelata—must serve as the starting-point. The iucurrent orifice (Fig. 209, o) homolo- gous "with a mouth, occupies the foremost region of the body, and leads into a rapidly widening space, triangular in section {k). The broad ventral surface is posteriorly somewhat expanded so that it forms two lateral cornua. These gradually narrow each into a tube-like continuation which breaks through the body-wall on the ventral surface {k') and fo
. Elements of Comparative Anatomy. ALIMENTARY CANAL OF TUNICATA. 399 Respiratory Chamber (Brancliial Sac). § 309. The simplest condition—that found in the Copelata—must serve as the starting-point. The iucurrent orifice (Fig. 209, o) homolo- gous "with a mouth, occupies the foremost region of the body, and leads into a rapidly widening space, triangular in section {k). The broad ventral surface is posteriorly somewhat expanded so that it forms two lateral cornua. These gradually narrow each into a tube-like continuation which breaks through the body-wall on the ventral surface {k') and forms a branchial aperture (spii-aculum). The dorsal continuation of the respiratory chamber is continued. Fig. 209. An Appendicularia seen laterally, n Nerve-centre, n' NerTe-chord. ot Ofcocyst. 0 Incurrent orifice, h Eespiratory chamber, e Ventral groove. / Ciliate tract, i Digestive canal, a Anus, h' Spiracle, t Testis, ov Ovary, c Root of the tail (after H. Fol). without any sharp line of demarcation into the beginning of the proper digestive canal ({). The two branchial apertures serving for the ejection of the water are cylindrical tubes, each of which takes its origin as an outgrowth of the wall of the respiratory chamber, into which an insinking of the outer body-wall pushes its way. The tube is beset with a ring of ciliated cells, which excite a streaming of the water directed at will now from the mouth through the respiratory chamber and the spiracula to the exterior, now in the reverse direction from without through the spiracula into the branchial pharynx towards the mouth. Mouth and spiraculum act here both as incurrent and excurrent orifices for the water. On the ventral surface of the respiratory chamber is found a deep groove, the ventral groove (e), open by small slits to the respiratory chamber. In front two bands of cilia (/) stretch from this groove towards the dorsal sui^face, embracing the entrance to the respiratory chamber. These structures are all co
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