250 GILBERT 0. BOURNE. 



section of the alimentary canal than is the case in other 

 simple Ascidians. Posteriorly its diameter gradually in- 

 creases, and it passes without any obvious line of demarcation 

 into the stomach. The stomach is flexed ventrally and has 

 the form of a wide^ U-shaped sac, occupying the posterior end 

 of the bod}'. The iutestiue takes its origin from the anterior 

 end of the distal or ventral limb of the stomach. On leaving 

 the stomach it passes into the ectoplerome of the left side of 

 the body and runs backwards and towards the dorsal side ; on 

 reaching the dorsal side of the proximal limb of the stomach 

 it turns forward and runs parallel with the oesophagus 

 towards the atrial siphon. The anus is situated on a conical 

 eminence in the atrium, opposite the atrial siphon. There is 

 no trace of a digestive gland or liver, but the distal limb of 

 the stomach is closely invested by the ramifications of the 

 testis. 



These organs may now be described in detail. 



The Branchial Sac. — This organ is extremely small as 

 compared with that of other Ascidians, its length being less 

 than one fifth of the whole length of the body. Its relations 

 are best understood by following a series of sections forward 

 from the region of the oesophagus. As is shown in figs. 3 

 and 34, the oesophagus is a relatively long and wide tube 

 lying on the dorsal side. Fig. 12 is a transverse section 

 taken a short way below the posterior limit of the branchial 

 sac. The oesophagus, invested by the endoplerome, is seen 

 on the dorsal side attached by the thick and short dorsal 

 suspensory fold to the body-wall. The ventral suspensory 

 fold is long and thin, and separates the right and left peri- 

 branchial cavities from one another. Fig. 11 is a section 

 through the extreme posterior limit of the branchial sac, and 

 shows that here the ventral suspensory fold has become much 

 thicker ; and its ventral moiety is channelled by a few 

 irregular passages which communicate with the right peri- 

 branchial cavity by elongated slit-like apertures or stigmata. 

 A few sections further forward similar passages are found 

 cumnuinicating by stigmata with the lett peribranchiul 



