238 



GILBERT G. BOURNE. 



to the exterioi' by the atrial apertui-o. The ventro-lateral 

 horns of this sac would push their way downwards right and 

 left of the gut to form the right and left peribranchial 

 cavities, and in so doing would divide the plerome into an 

 inner moiety surrounding the gut, which we may call the 

 endo plerome, and an outer moiety forming with the external 

 ectoderm the wall of the body. This latter moiety is what 

 is usually called the mantle; I propose to call it the ecto- 



At.o. 



D.SUS 





V.sag 



1. — Section of a liypoflieiical Tunicate larva before tlie formation 

 of the atrial and pcribraiicliial cavities. The mesoderm (plerome) is 

 a solid mass of tissue in which the gonads and tlie heart enclosed in 

 its pericardium are embedded. 



2. — Diafrrammatic transverse section throngli the ])harynx of an 

 Ascidian showing the plerome divided into an outer layer (the ecto- 

 pleronie) and an inner layer (the endoplcrome), surrounding the 

 pliarynx. 



3. — Diagram of a section through the oesophagus of Oligotrema 

 psanimites. At. o. Atrial ajjcrture. CIi. Notochord. D. sus. 

 Dorsal suspensory fold. Ecp. Ectopleronie. Fnp. Endoplerome. En. 

 Enteron. Go. Gonads. Bt. Heart. N. Nerve-cord, Oe. (Esophagus. 

 Pi^. Peribranchial cavities. Pc. Pericardium. P//. Pliarynx. V.sus. 

 Ventral suspensory fold. The ectoderm is represented by a thick 

 broken line ; the plerome (mesoderm) is shaded with dots. 



plerome. It contains the gonads, renal organ, lieart, 

 intestine, and other viscera. On the formation of the bran- 

 chial slits the endoplerome of the branchial sac is, of course, 

 broken up, and is represented by the mesoblast of the 

 branchial bars, as indicated in the diagram in the text. Fig, 

 2. In most Ascidians, owing to the gr(>at size of the branchial 

 sac, the endoplerome is practically reduced to the blood- 



