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. 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 connected with 

 the taking in of food. 



