Fig. 3

Internal anatomy of Iosactis antarctica sp. nov. a Longitudinal section of the distal column showing the endodermal marginal sphincter muscle and the sphincter in the base of the tentacle (arrows); note the well-developed circular musculature in the column. b Cross section of the column through the mesenteries at the level of the actinopharynx showing the cycles of mesenteries; numbers between pairs indicate the cycle of mesenteries. c Detail of the sphincter of a tentacle; note the muscle fibres along the entire base of the tentacle. d Detail of the directive mesenteries, the retractor and the parietobasilar muscles. e, f and h Detail of the marginal column showing the endodermal diffuse to moderately circumscribed marginal sphincter in different specimens. g Longitudinal section of the proximal column showing the invaginated pedal disc (arrows). i Cross section of a tentacle. j Detail of a longitudinal section of the proximal end showing the well-developed basilar muscles. ga gastrodermis, me mesoglea, s siphonoglyph. Scale bars a, c, d, e, f, h, i, j 0.5 mm; b, g 2 mm