Fol, 1872
Trunk elongate, up to 1.5 mm long (Oikopleura fusiformis). Postcardial caecum large, slightly curved, pointing dorsally and genitally, tapering terminally. Rectum extending beyond the oral wall of the stomach. Gonads adjoining the caecum, initially genito-ventrally, subsequently also laterally, and surrounding the stomach when ripe. Tail musculature narrow.