40 



THE VOYAGE OF H.M.S. CHA.LLENGER. 



In the AppendiculariicUe the peribranehial cavity is represented by two "atrial canals," 

 which connect the two stigmata of the branchial sac with the exterior. They lie on the 

 ventral side of the sac at the sides of the endostyle, and the apertures are near the middle 

 of the ventral surface close to the anus. A similar condition of parts is found at a 

 comparatively late stage of development in Simple Ascidians, and the single peribranehial 

 cavity of the adult is the result of the fusion of the two atria, into one of which the 



■g.«L 



l].s. 



... ty. 



v. v. ' ni.T). 



Fiq. 9. — Diagrammatic transverse section through the middle of the body of Ascidin. 



t.,test; ec. , ectoderm ; >«., mantle; m.b., muscle-bands in mantle; U.S., blood sinus in mantle ; w.v., great ventral vessel ; mt.e., epithelium 

 lining the peribranehial cavity ; p.br., peribranehial cavity; cl., cloaca; br.s., cavity of branchial sac ; l.v., longitudinal vessel of bran- 

 chial sac; sg., stigma ; ere., endostyle ; br.s.e., epithelium lining branchial sac ; d.l., dorsal lamina; con., connective between branchial 

 sac and mantle; L, section of intestine ; ty., typhlosole ; ov., ovary; r.o., renal organ ; r., section of rectum; g.d., section of genital 

 ducts. 



anus has come to open. The common aperture finally moves to the dorsal surface of 

 the body ; consequently the dorsal atrial aperture of the Ascidian represents the 

 three ventral apertures of Appendicularia. 



