Order III. Cestoidea.
In this order the body is so much compressed in the transverse plane and elongated in the sagittal plane that it assumes the shape of a long narrow band or ribbon. The tentacular sheaths are present but the tentacles are degenerate in the adult. The tentacular functions are performed by numerous tentilla situated in long grooves extending along the whole length of the oral side of the band-like body. The transverse ribs are reduced; the sagittal ribs extend along the whole of the aboral side.
Fam. Cestidae.—This is the only family of the order. Cestus veneris, the Venus's girdle of the Mediterranean Sea, is also found in the Atlantic Ocean, and specimens belonging to the same genus, but probably to a different species, occur as far north as the White Sea. Some of the larger specimens are considerably over 1 metre in length.
Fig. 182.—Cestus pectenalis. Ab, aboral sense-organ; Ct, the sagittal ribs; M, mouth. (After Bigelow.)
C. pectenalis was found in abundance off one of the Maldive Islands [[431]] and differs from C. veneris in having a large and prominent orange patch at each end of the body. It is said to be extremely graceful in the water, moving with slow, ribbon-like undulations, and shining in the sunlight with a violet iridescence. Vexillum, from the Mediterranean Sea and Canary Islands, is rather more pointed at the extremities than Cestus, and differs from it in some important anatomical characters.
Order IV. Platyctenea.
This order has been constituted for two remarkable genera, in which the oro-apical axis is so much reduced that distinct dorsal and ventral surfaces can be distinguished.
There is a single pair of long milky-white tentacles capable of complete retraction into tentacular sheaths.
Fam. 1. Ctenoplanidae.—Ctenoplana was discovered by Korotneff in 1886 floating with the Plankton off the coast of Sumatra. In 1896 Willey [[432]] discovered four specimens on a cuttle-bone floating off the coast of New Guinea. To these authors we are indebted for the only accounts of this animal that have been published.