TUNICATA. 15 
MoLcuLA CONCOMITANS. 
(Plate V., figs. 1 A, and 2-7.) 
Locality. —Winter Quarters, in McMurdo Bay. The single specimen measures :— 
Length 2°5 cm., breadth 2 cm. 
External Appearance.—Body somewhat globular, rather flattened laterally, with a 
straight anterior and a rounded posterior end. Apertures at the ventral and dorsal 
edges of the anterior end, both on well-marked siphons, the atrial being the more 
prominent (Plate V., fig. 1A). The surface is not encrusted with sand, but has 
small tag-like excrescences scattered over it, especially around the siphons. Colour 
grey. 
Test thin, cartilaginous, translucent ; prolonged into minute processes connected 
with the vessels of the test, and bearing occasional foraminifera or minute grains of 
sand, especially about the anterior end. 
Mantle yellow, opaque, and very muscular (fig. 4)—the sphincters being 
especially strong. The mantle adheres closely to the test. 
Branchial Sae with seven folds on the right side and six on the left. There are 
seven bars on a fold, and one large, with several imperfect smaller bars, in the 
interspace. Stigmata not much curved, irregularly placed, varying considerably in 
length (see fig. 5). 
Dorsal Lamina a short plain membrane. 
Tentacles, eight very large and much branched, with some much smaller ones 
placed irregularly between. 
Dorsal Tubercle large and simple, horseshoe-shaped, with the horns turned 
inwards (fig. 6). 
Alimentary Canal bulky, intestine forming a narrow dark-coloured loop. 
Gonads large and yellow, a single sausage-like mass on each side. 
The single specimen of this species belongs to the group of Molgulids with a nearly 
naked test, not covered with adhering sand and gravel. In this respect it resembles 
M. citrina, M. nuda, M. ampulloides, and M. helleri from the Northern hemisphere, 
and M. maxima and M. pedunculata from Southern seas; but it differs from all of 
these in details of anatomy. It comes, perhaps, nearest to AZ maaima (described by 
Professor Sluiter from the ‘ Charcot’ collection) ; and, in fact, it closely resembles that 
Antarctic species in external appearance and in several other respects. It differs, 
however, notably in the mantle. Professor Sluiter describes M. maxima as having 
the mantle feebly developed, with a feeble musculature; whereas our specimen has an 
extraordinarily strong and opaque mantle, with conspicuous yellow muscles (fig. 4), 
like those of a Microcosmus. The atrial siphon is the longer and narrower, the branchial 
being short and wide. The large tentacles are extraordinarily bushy. 
The branchial sac, although agreeing in some respects, such as the number of bars 
