C. ROUSSELET ON PEROPHORA LI8TERI. 333 



force, and eject water from its oral and anal orifices. Within 

 this second tunic there is also seen an extensive network of 

 blood sinnses, through which the blood circulates freely. 



The oral or ingestive aperture leads into a wide chamber 

 known as the branchial sac, which fills nearly the whole of the 

 test. Its walls are perforated by four rows of minute elongated 

 slits fringed with cilia, and by the action of these cilia a 

 current of water is produced which enters the oral aperture, 

 passes through the openings in the wall of the sac, and out at 

 the anal orifice. The walls of the branchial sac are strengthened 

 by minute rod-like thickenings of the integument, lying at 

 right angles to each other, and these contain hollow spaces 

 through which the blood circulates, and by means of the con- 

 stant flow of water oyer them the blood becomes aerated. Thus 

 the function of the sac is clearly seen to be respiratory. 



On the other hand the branchial sac is nothing else but the 

 anterior part of the alimentary canal, and therefore corresponds 

 to the pharynx of other animals ; and in being perforated and 

 subserving the function of respiration it is unlike the corres- 

 ponding organ in the invertebrata, but analogous to the per- 

 forated pharynx of Amphioxus and the lower fishes. 



At the bottom of the branchial sac there is an opening which 

 leads by a short oesophagus into the stomach, from which an 

 intestine is continued in an S-shaped curve. It lies at the side 

 of the branchial sac, and ends in an anus, which, however, does 

 not communicate directly outwards, but opens in the middle of 

 the atrial chamber. 



The branchial sac is only connected along one side (the 

 haemal side) and around the mouth with the test and mantle ; 

 it otherwise lies quite free within the tunic, a few pillars of 

 muscular or connective tissue only serving to keep it in place. 

 A wide space is thus left nearly all round, which is known as 

 the atrium or atrial chamber, and it is with this chamber that 

 the excretory or anal aperture of the test communicates. 



The atrium, according to Professor Huxley, is lined by a 

 membrane which is reflected like a serous sac on the pharynx 

 and viscera, and constitutes a third tunic or peritoneum. It 

 enters into close union with the proper wall of the pharynx, 

 and is perforated with it by the ciliated slits. 



If this be correct, it will be observed that the branchial sac, 



