TUNIC AT A. 13 





 MOLGULA BACCA. 



(Plate IV., figs. 1-5.) 



Locality. Winter Quarters, in McMurdo Bay. A single specimen, measuring in 

 length of body 2 '2 cm., breadth 1 '6 cm., length of stalk 0'4 cm. 



External Appearance. Body pyriform, nearly globular, attached by a short 

 posteriorly placed stalk (Plate IV., figs. 1, 2). Anterior end rounded, bearing both 

 apertures, which are small, but distinct. The surface is absolutely smooth and shining. 

 Colour pale yellowish grey. 



Test thin as tissue paper, membranous and transparent. A few foraminifera and 

 minute sand grains adhere to the test round the atrial aperture. 



Mantle thin, but with delicate muscle bands and well-marked sphincters. 



Branchial Sac very delicate, with seven folds on each side. There are five or six 

 bars on a fold, and three between. Many of the stigmata are long and narrow, or 

 nearly straight especially those next the endostyle and in the middle of the inter- 

 spaces ; while they are coiled in well-marked infundibula in the branchial folds (fig. 3). 



Dorsal Lamina a plain narrow membrane. 



Tentacles branched, eight large and eight small, placed alternately, with a number 

 of still smaller ones between (fig. 5). 



Dorsal Tubercle narrow, elongated antero-posteriorly (fig. 5), with the opening 

 directed laterally and both horns turned inwards. The tubercle lies upon the nerve 

 ganglion. 



Alimentary Canal showing through the test upon the left side of the body. The 

 intestine is long and narrow and makes a circular loop. The rectum is closely adherent 

 to the stomach (fig. 4). 



This species ia closely related to the last, but differs so remarkably in external 

 appearance and in a few other points that it seems impossible to regard them as the same. 

 The body is here very much more globular and the stalk is much shorter. The two 

 apertures are equally anterior, and the siphons are relatively smaller. The single 

 specimen was tensely filled with fluid, but when pierced at once collapsed to a flabby 

 membrane. In the living state it was probably distended with sea water and nearly 

 globular in form. 



Our present species shows some superficial resemblance to the Molgula chrystallina 

 of H. P. C. Holier, the Pera pellucida of Stimpson, but differs considerably in internal 

 organisation, as that northern species has only five branchial folds on each side, while 

 the present southern one has seven. Moreover, there are larger numbers of bars on the 

 folds here otherwise the branchial sacs are rather alike in their details. The dorsal 

 tubercles in the two species are somewhat similar, and the course of the alimentary canal 

 is the same in both. All this is rather curious, and suggests that we have in M. bacca 

 a representative species to M, chrystallina of the northern hemisphere ; but I must 



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