Order III.—Larvacea.
Fig. 24.
Oikopleura cophocerca in its “house” (after Fol); seen from right side, × 6. Arrows indicate course of the water; x, lateral reticulated parts of the “house.”
The Larvacea are very minute Tunicata which live at the surface and swim by means of a tail-like appendage, resembling in this and certain other respects the tadpole larva of other Tunicata. They are able to form a temporary test or “house” many times larger than the body (Fig. 24). The organism itself, which is almost lost in its large test, is the little hammer-shaped body in the centre of the figure; the streaked areas bound a space in which the tail lashes vigorously. The animal can leave its test and secrete another in a few hours.
The tail is attached to the under or ventral surface of the tiny little barrel-shaped body, and usually points forwards; a skeletal rod, the urochord, runs along its length. The branchial sac has two ciliated openings or gill-clefts leading directly to the exterior, and not opening like the stigmata of the other orders into an atrial cavity.
The order contains one family, the Appendiculariidæ, and four genera, and is represented in all seas.
Oikopleura cophocerca, one of the largest forms, is about half an inch in length. The exhibited specimens came from St. Andrews, Fife. Professor McIntosh reports that occasionally specimens of this species occur in immense quantities, the tow-nets being filled with them.