FAM. 4. MOLGULIDAE.—Solitary sessile Ascidians, sometimes not fixed; branchial aperture six-lobed, atrial four-lobed. Test usually encrusted with sand, which is generally attached to branched hair-like processes from the test (Fig. 40, C). Branchial sac longitudinally folded; stigmata more or less curved, usually arranged in spirals (Fig. 41); tentacles compound. The chief genera are—_Molgula_, Forbes (Fig. 40, C), with distinct folds in the branchial sac (Fig. 41, B), and _Eugyra_, Ald. and Hanc., with no distinct folds, but merely broad internal longitudinal bars in the branchial sac (Fig. 41, C). In some of the Molgulidae {78}(genus _Anurella_, Lacaze-Duthiers), the embryo does not become converted into a tailed larva, the development being direct without metamorphosis (see Fig. 42, C). The embryo when hatched gradually assumes the adult structure, and never shows the features characteristic of larval Ascidians, such as the urochord and the median sense-organs. Fig. 42 shows an Ascidiid (A), a Cynthiid (B), and this exceptional Molgulid (C), type of larva, and three forms of Compound Ascidian larvae, the Distomatid (D), the Botryllid (E), and the Diplosomatid (F). [Illustration: FIG. 42.—Larvae of various Ascidians. A, _Ascidia mentula_, Linn.; B, _Polycarpa glomerata_, Alder; C, _Anurella roscovita_, Lac.-Duth.; D, _Distaplia magnilarva_, Della Valle; E, _Polycyclus renieri_, Lamk.; F, _Diplosomoides lacazii_, Giard. (Mostly after Lahille.)] In the Molgulidae the viscera are characteristic in position and appearance. The alimentary canal lies on the left side of the branchial sac, and the intestine forms a long narrow loop directed in the main transversely. The pericardium and heart are on the middle of the right side, and behind them is placed the single sac-like ductless renal organ, generally occupied by one or more concretions. The gonads are in most cases on both sides of the body, in front of the intestine on the left, and in front of the heart on the right; but in _Eugyra_ there is no gonad on the right side, and in some other forms the gonad on the left side is absent. (For _Oligotrema_, see p. 111, note.) There are a number of British Molgulidae, the two commonest {79}of which are—_Molgula oculata_, Forbes, thickly covered with gravel or broken shells, and forming an ovate mass as large as a walnut; and _Eugyra glutinans_, Möller, a smaller more globular body, the size of an acorn, and covered with fine sand, except at one circular area near the posterior end, where the leaden grey test shows through. Both these species are obtained by dredging in from 10 to 30 fathoms, and lie freely on the bottom. A rather rarer littoral species _Molgula citrina_, Hancock, found on some parts of our coast (_e.g._ in the Firth of Forth, at Arran, and at Port Erin), is exceptional in having the test free from sand, and in being fixed like an _Ascidia_, generally to the lower surfaces of large stones near low tide.