REPORT ON THE TUNICATA. 



33 



others different sizes alternate. The second set of vessels, the fine longitudinal or 

 interstigmatic, are on the same plane as the transverse vessels, but run longitudinally, 

 separating the different stigmata in a row, and serving as a means of communication 

 between the transverse vessels (figs. 4 and 5, l.v.). These interstigmatic vessels are some- 



FlQ. 4. — A Single Mesh of the Branchial Sac of Ascidia, seen from the inside. 

 tr., transverse vessel ; i.l., internal longitudinal bar ; l.v., fine longitudinal vessel ; p., papilla ; jf., smaller intermediate papilla : 



c.d., connecting duct. 



times very irregular. The third set of vessels, the internal longitudinal (figs. 4 and 

 5, i.l), occupies a plane internal to the first and second, and consists of a series of stout 

 bars running from the anterior to the posterior end of the branchial sac at right angles 

 to the transverse vessels and communicating with them by short wide connecting ducts 



l.v. 



Fio. 5.— Diagrammatic horizontal section through a Mesh of the Branchial Sac of Ascidia, showing the interior of the transverse 



vessel, connecting duct, and papilla at the right hand end. 

 tr., transverse vessel ; l.v., fine longitudinal vessel ; i.l., internal longitudinal bar ; p., papilla ; c.d., connecting duct ; 



h.m., horizontal membrane. 



(figs. 4 and 5, c.d.) placed at the points of intersection. These strong bars are a prominent 

 feature in the branchial sac viewed from the interior, and, along with the transverse 

 vessels, form the boundaries of the rectangular meshes into which the inner surface 

 of the branchial sac is divided. 



