280 



TUNICATA. 



mus, it occupies all the length and the breadth of one of the 

 sides, and the rest of the viscera occupies the other side : 

 Fig. 48. then its form is oblong, oval, or 



rectangular. Sometimes, as in 

 Ascidia mammillata and monachus, 

 after having descended even to the 

 bottom of the outer tunic, it bends 

 upwards until its base is at the 

 middle of its length, and looks to- 

 wards the entrance. In the latter 

 case, the parietes have the greatest 

 extent. In general these are 

 smooth and without plaits ; but 

 in some species, and, as it would 

 appear, in all those which have a 

 coriaceous outer cloak, they are 

 creased into deep and regular folds, 

 the first vestiges of the four branch- 

 ial leaves of bivalves. 



Whatever may be, however, the 

 shape and general disposition of 

 the sac, the texture of its inner 

 parietes remains essentially the 

 same, and is so very remarkable 

 that several authors, who knew not its purport, have ex- 

 pressed astonishment at its beauty. It consists of an infinity 

 of little vessels which cross one another at right angles, and 

 thus weave a network with quadrangular meshes (Fig. 48), 

 that are again subdivided by vessels of such tenuity that 

 they elude the unaided vision, and require the microscope 

 for their discovery. With a little attention it may be per- 

 ceived that the vertical vessels come from the transverse 

 vessels, and that these are connected by their extremities 

 to two great trunks, also vertical, which occupy one of the 

 sides, or rather the edges, of the sac ; and it is natural to 

 conclude that one of these trunks is the artery, and the 

 other the branchial vein.* 



The meshes of this branchial network are generally, as I 

 have said, nearly square and uniform, yet in the different 

 genera there is exhibited a considerable variety of patterns, 

 some of which you have here copied from the beautiful 

 plates of Savigny. Fig. 49 exhibits a small portion of the 



Phallusia sulcata of Savigny, 

 opened. 



* Cuvier, Mem. xx. 11, 12. Spallanzani mistook the vessels for minute 

 muscles, "the longitudinal to shorten by their action the length of the 

 body, and the transversal to contract the breadth." Travels, iv. 269. 



