268 (iiLBEkT V. ];onuNK. 



Crustacea of large size relatively to itself, and is no longer 

 dependent on minnte organisms and organic debris swept 

 into its branchial chamber by ciliary cnrrents and there 

 filtered off from tlie water by an extensive branchial 

 apparatus. Hence we find that the branchial sac^ being 

 no longer of prime importance for the nutrition of the 

 animal, has undergone great reduction in size, as also have 

 all those accessory organs whose function it is to secrete 

 strings of mucus for the entanglement o£ the solid matter 

 brought in by the branchial current and to direct the food by 

 ciliary action along definite tracks to the oesophageal opening, 

 viz. the endostyle, the peribranchial grooves, and the dorsal 

 lamina. In consequence of the reduction in size of the 

 branchial sac, and therefore of the stigmatic area, a relatively 

 small current of water can pass through its meshes into the 

 atrial cavity, and the reduction in size of the atrial siphon 

 follows almost as a matter of course. On the other hand, the 

 oesophagus and stomach are relatively f:ir better developed 

 than is the case in other simple Ascidians, and the dorsal 

 oesophageal groove, facing the oblique elongated mouth- 

 opening, becomes a prominent feature, being clearly adapted 

 to transmit the organisms swallowed as food directly into the 

 oesophagus. 



In conclusion I must express my thanks to Professor 

 W. A. Herdman, who was kind enough to read through my 

 rough manuscript and to make many valuable suggestions, 

 which I have incorporated in this paper ; also to Professor 

 Weldon, who, while I was working in his laboratory, assisted 

 me from time to time in the solution of difficult problems of 

 structure and histology. Most of all, my thanks are due to 

 Dr. A. Willey, who, inadvertently perhaps, sent me the 

 specimen of Oligotrema, which has been a subject of much 

 interest to me. 



