LIVING MATERIAL

Members of this species are sometimes gathered, together with individuals of M. manhattensis. from Eel Pond at Woods Hole, Mass. While their anatomy is almost identical, the two species are radically different in their mode of reproduction. M. citrina is viviparous, M. manhattensis oviparous.

Mid-June to mid-September (Grave, 1926). The period of reproduction seems to depend on the size of the individual rather than on the time of year.

A. Care of Adults: Sexually mature individuals (those over five mm. in length) will continue to reproduce and release larvae when brought into the laboratory, if they are kept in vessels supplied with a gentle stream of sea water.

B. Procuring Gametes: It is apparently not possible to remove and then fertilize ovarian eggs of this species.

C. Preparation of Cultures: Rearing embryos outside the parent is difficult. Berrill (1935) suggests that they be raised in a thistle-tube, the large end of which is covered with bolting silk on which the eggs are placed, the other end being attached to a T-tube through which air is bubbled. The whole apparatus is submerged in sea water; further details can be found in the original article. Even with this set-up, the mortality rate is high, and it is better to take the stages desired for study directly from the atrial brood-chamber of the parent.

Larvae are released when the adults are exposed to light; no particular time of day seems to be optimal. Culture directions for the tadpole are similar to those given for M. manhattensis (see p. 216). This form is favorable for use in a study of metamorphosis.

A. The Unfertilized Ovum: The egg measures approximately 210 microns in diameter, and is very opaque due to the presence of a yellow-orange pigment in the densely packed yolk. The outer follicle cells form a markedly flattened layer over the surface of the chorion, while the inner follicle cells are closely packed into a narrow perivitelline space (see the paper by Berrill, 1931, Figure 2). The egg is shed from the oviduct at the metaphase of the first maturation division.

B. Fertilization and Cleavage: Fertilization and development take place within the atrial chamber of the parent. The first few cleavages are equal, dividing the egg into a right and a left half. Gastrulation is by invagination, between the sixth and seventh cleavages (Berrill, 193S).

C. Rate of Development: Details are not available, but development in this form is relatively slow. At 16û C., the tadpoles hatch by rupturing the chorion 150 hours after fertilization.

D. Later Stages of Development and Metamorphosis: The free-swimming larvae, although they bear the same superficial resemblance to urodele larvae as do those of Amaroucium or Botryllus, seem to be less highly specialized. They lack organs for attachment, and no gill slits are visible. The most conspicuous larval organ is a huge sensory vesicle containing a statolith; there is no "eye." The alimentary tract is very yolky and poorly differentiated. The atrium consists of two sacs joined dorsally and posteriorly; the siphons are inconspicuous. The tail-fins are vertical. A small bilobed pericardial sac lies anterior and ventral to the intestine. Eight thickenings in the mantle precede the formation of the ampullae (see the paper of Grave, 1926, for a diagram).

The free-swimming period is short, averaging less than three hours (Grave, 1926). Fixation can be accomplished at any region of the adhesive test, and is accompanied by tail} shrinkage and the extension of 8 or 10 mantle projections, the ampullae. A very small percentage of larvae metamorphose within the egg membrane inside the atrial cavity. Details and diagrams of metamorphosis are available in papers by Grave (1926) and Berrill (1931).

BERRILL, N. J., 1931. Studies in tunicate development. II. Abbreviation of development in the Molgulidae. Phil. Trans. Roy. Soc., London, ser. B, 219: 281-346.

BERRILL, N. J., 1935. Studies in tunicate development. III. Differential retardation and acceleration. Phil. Trans. Roy. Soc., London, ser. B, 225: 255-326.

GRAVE, C., 1926. Molgula citrina (Alder and Hancock). Activities and structure of the free swimming larva. J. Morph., 42: 453-471.