70 THE VOYAGE OF H.M.S. CHALLENGER. 



The Branchial Sac is very thick and strong, with seven large folds on each side. 

 The transverse vessels are very large, few, and visible to the eye. They give off numer- 

 ous branches, which divide and anastomose, forming a network of wide vessels which form 

 the meshes of the sac. These vessels give off much smaller tubes which bound the 

 stio-mata. The stigmata are numerous, irregular, and arranged in spirals forming 

 infuudibula. 



The Dorsal Lamina is a short but very broad membrane, with no ribs, but having 

 an irregular margin. 



The Tentacles are compound, and large ; they are sixteen in number, and of two 

 sizes, placed larger and smaller alternately. 



The Dorsal Tubercle is large and prominent, with both horns much coiled spirally. 



This large species was obtained by Professor E. 0. Cunningham in the Straits of 

 Magellan during the cruise of the " Nassau," and was briefly described under the name of 

 Cynthia gigantea in the Transactions of the Linnsean Society of London for 1871. 



Cunningham's description merely refers to a few features in the external appear- 

 ance, and is insufficient for the identification of the species. It was only when 

 examining the " Nassau " collection in the British Museum that I discovered that the 

 large Challenger Molgula from the Straits of Magellan was identical with Cunningham's 

 Cynthia gigantea, which had been referred to the wrong genus and family ; consequently, 

 while retaining the old specific name, I have treated it otherwise as a new species, and 

 have figured and described it anew in detail. 



The external form is somewhat variable ; some specimens are almost globular, and 

 others fusiform, while all intermediate shapes occur. The specimen figured (PL IV. fig. 1 ) 

 is typical as to shape, but is of small size. Cunningham mentions that the largest 

 specimen obtained by the " Nassau " measured " eight inches from base to apertures, 

 and was between four and five inches broad." Many of the Challenger specimens exceed 

 those dimensions, the largest being 33 cm. in length, and 17'5 cm. in breadth. This is 

 the next largest Simple Ascidian to Ascopera gigantea. 



The following list shows the proportions between length and breadth in thirty of the 

 Challenger specimens : — 



