256 GILBERT 0. BOURNE. 



canals and chambers, which recall, more than anything else, 

 the inciirrent and excnrrent canals of a sponge. The cham- 

 bers open on the one hand by wider or narrower orifices into 

 the cavity of the branchial sac, and on the other hand they 

 communicate by means of numerous stigmata with the peri- 

 branchial cavities. The arrangement of the chambers and 

 stigmata is so complicated that it is difficult either to figure 

 or describe it, but some idea of their relations may be gained 

 by a study of fig. 16, which represents the portion of the 

 branchial wall marked z in fig-. 9, as seen from the inner or 

 branchial surface. The drawing is made from a model recon- 

 structed from about thirty serial sections, and shows the 

 loAver moieties of the two incuri'ent branchial chambers 

 separated by a stout vertical fold of the branchial wall ; this 

 fold probably corresponding to one of the folds in the 

 branchial sac of Molgula, while the incurrent chamber is a 

 highly modified form of a cavity of a fold. Each chamber is 

 subdivided in a most complicated way by cross-partitions and 

 trabecula3 into a number of secondary chambers (fig. 26, ic~), 

 wliicli are prolonged upwards and downwards far beyond the 

 limits of the openings into the primai'y incurrent chambers. 

 Both primary and secondary incurrent chambers open by 

 numerous elongate stigmata — some of which are clearly shown 

 in fig. 16 — into the peribranchial cavity, the walls of which 

 are in places so deeply infolded as almost to form a set of 

 excurrent chambers into which the stigmata open. Some 

 idea of the complexity of these folds and chambers may be 

 gathered from fig. 26, which is a drawing of one of the 

 sections included in the middle of the model. The stigmata 

 are fairly numerous and for the most part irregularly 

 ari'anged, but they tend to rise one above another in a 

 scalariform manner, as shown on the left hand of fig. 16. 

 There is no trace of a spiral arrangement. The right-hand 

 incurrent chamber of the model has ten stigmata opening 

 from it, and the sections above and below the limits of the 

 model show that about as many more open into the peri- 

 branchial cavity froin its anterior and posterior piolongations. 



