20 



THE VOYAGE OF H.M.S. CHALLENGER 



and one of them, placed in the mid-ventral line, is always considerably larger than the 

 rest (PI. I. fig. 2). 



The branchial sac occupies about the middle two-fourths of the length of the 

 Ascidiozooid. It is the widest part of the body. The stigmata are very conspicuous ; 

 they are transversely directed slits extending along the whole breadth of each side of 

 the sac, and separated by the transverse vessels. These stigmata, therefore, correspond 

 not to the stigmata of the ordinary Simple and Compound Ascidians, such as the 

 species of Ascidia or of Botryllus, where the slits are elongated antero-posteriorly, 

 and are separated not by the transverse but by the fine longitudinal vessels (Fig. 3, A), 

 but to the rows of stigmata which lie between the transverse vessels, and are divided 



-^..U 



a- 



-mh. 



bJ 





Q) 



A 



B 



Fig. 3. — Diiigrams showing the structure of the branchial sac in : — 

 A. Ascidia, B. CuIeoluSt and C. Pyrosmna, 



i.l. internal longitudinal bar, Lv. fine longitudinal or interstigmatic vessels (in Ascidia only), 

 mli. mesh, ^g. stigmata, tr. tr'. transverse vessels. 



into meshes by the internal longitudinal bars, — one complete mesh, divided into six 

 stigmata, is shown in Ficr. 3, A. 



In the genera Culeolus and Fungulus and Bathyoncus, however, amongst Simple 

 Ascidians,^ and in Pharyngodictyon amongst Compound Ascidians,'' — all of them deep- 

 sea genera, made known through the Challenger investigations, — we find a condition of 

 the branchial sac similar to that of Pyrosoma. In these forms there are no fine 

 longitudinal vessels, and consequently the meshes formed by the intersection of the 

 transverse vessels and internal longitudinal bars are not cut up into stigmata, but 

 remain as large quadrangular spaces (Fig. 4, B). If, now, the transverse vessels become 

 more numerous and more closely placed, so as to reduce the quadrangular meshes to 

 transverselj- elongated slits, we arrive at the condition found in Pyrosoma (Fig. 4, C). 

 In Culeolus and its allies, in Pharyngodictyon, and in Pyrosoma, there are therefore 

 no true stigmata, and no interstigmatic vessels, such as those of Ascidia, but merely 

 meshes bounded by the transverse vessels and the internal longitudinal bars. 

 1 See Part I. of this Report. ^ gge Part II. of this Report. 



