140 LILIAN SHELDON. 



(figs. 12 and 15, Ps. p.) project into the atrial cavity along its 

 outer wall, being only separated from it by the epithelial 

 lining of the atrial cavity (figs. 12 and 15, A. Ep.) which 

 covers them. Figure 12 is a diagrammatic transverse section 

 through Cynthia, and represents the relation of these processes 

 to the atrial cavity. They appear very conspicuously when 

 the atrial cavity is opened, especially in the fresh animal. 

 They are then seen as very large, opaque, white bodies, which 

 fill up a considerable portion of the cavity. I am not aware 

 that any analogous structure has hitherto been described in 

 any Ascidian. 



The connective tissue extends from the body wall inwards 

 to the atrial epithelium (fig. 12, Ps.), and also surrounds the 

 alimentary canal. There is therefore no body cavity, its place 

 being occupied by what appears to be a system of sinuses, the 

 third kind of cell described above being the blood-corpuscles. 

 For convenience of description I shall hereafter speak of the 

 sinuses as the pseudoccele ; but I do not wish to imply that it 

 is necessarily homologous with the so-called pseudoccele of 

 some Invertebrates. There are no definite blood-vessels, the 

 heart opening out at both ends into the sinuses. Strands also 

 pass across the atrial cavity to the pharynx, the spaces in the 

 branchial bars being continuations of the sinuses, plentifully 

 supplied with blood-corpuscles (figs. 5 and 7, br. b.). 



It seems probable that the function of the processes pro- 

 jecting into the atrial cavity is to expose a greater surface of 

 the sinus to the influence of the oxygen contained in it. 



The Atrial Cavity. 

 The atrial cavity is very capacious, extending beyond the 

 posterior end of the alimentary canal. For the greater part 

 of its extent it is divided completely into two halves by a par- 

 tition, which passes along from the outer wall of the atrial 

 cavity to the walls of the pharynx. This partition starts just 

 behind the buccal cavity, follows the line of the endostyle, and 

 curves round the posterior end of the pharynx on to its dorsal 

 surface, where it passes along the line of the dorsal lamina, 



