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 (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).
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 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.
Specific Characters and Dorsal Tubercle.—The chief points in which the various genera and species of Simple Ascidians differ are the details of the branchial sac (see Figs. 22, 35, 36, 38, and 41), the condition of the tentacles (Fig. 37), the dorsal lamina or languets, and the dorsal tubercle, in addition to form, colour, and other external features.
Fig. 43.—Various forms of dorsal tubercle in Simple Ascidians. 1. Molgula pyriformis; 2. Forbesella tessellata; 3. Ascidia meridionalis; 4. Cynthia formosa; 5. Cynthia papietensis; 6. Ascidia challengeri; 7. Polycarpa tinctor; 8. Cynthia cerebriformis; 9. Ascopera gigantea; 10. Boltenia tuberculata; 11. Ascidia translucida; 12. Culeolus moseleyi; 13. Ascidia pyriformis; 14. Boltenia pachydermatina; 15. Microcosmus draschii; 16. Styela etheridgii; 17. Styela whiteleggii; 18. Polycarpa aurata. (After Herdman.)
Fig. 43 shows some of the more remarkable forms of dorsal tubercle. Starting with a simple circular opening (1) surrounded by a thickened ciliated ring, the anterior border becomes pushed in to form a crescentic slit (2 and 3). The horns of the crescent then grow longer and may be turned in (4 and 5) or out (6 and 7), and so give rise to the many varieties of horse-shoe (such as 6), perhaps the commonest form of dorsal tubercle in Simple Ascidians. In many Cynthiidae the central part of the horse-shoe remains small, while the horns become long and much coiled so as to constitute two prominent spirals (8, 9, 10). In other exceptional forms again the curved slit becomes straightened out, undulating (11), irregularly bent (12 and 13), elaborately folded (14 and 15), or broken up into pieces (16), so that there come to be several or even a large number (17 and 18) of minute openings in place of the original single aperture.
It cannot be said that any form of dorsal tubercle is characteristic of any of the families or genera of Ascidians, and in the case of some species the organ is liable to great individual variation; but still in most species there is found to be a characteristic shape or appearance of tubercle which is a useful diagnostic feature.