XV TENTACULATA — CESTOIDEA PLATYCTENEA 42 I 



minent oraage patch at each end of the body. It is said to be 

 extremely graceful in the water, moving with slow, ribbon-like 

 undulations, and shiniag 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. Otenoplanidae. — Ctenoplana was discovered by 

 Korotneff in 1886 floating with the Plankton off the coast of 

 Sumatra. In 1,896 Willey^ discovered fom- 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. 



When the Ctenoplana is creeping on the bottom of a dish 

 or with its dorsal side downwards on the surface film of the 

 water, it has the form of a flattened disc with a notch on each 

 side. On the upper or dorsal surface eight short rows of cteno- 

 phoral plates may be seen, and in a position corresponding with 

 the two notches in the margin of the body are situated the two 

 sheaths from which the long pinnate tentacles protrude. In the 

 exact centre of the dorsal surface is situated the statolith, 

 supported by stiff processes from adjacent cells ; and forming a 

 circlet round the statolith there is a row of short ciliated tentacles. 

 These tentacles, however, when examined carefully in the living 

 animal, are found to be arranged in two sets of about nine in 

 each, separated by narrow gaps on each side, the gaps corre- 

 sponding in position with the axis through the tentacles. 



When the animal is swimming it assumes a helmet-shape by 

 depressing the sides of the body like a pair of flaps on the 

 tentacular axis, and then the ctenophoral plates come into play and 

 produce the progressive movements of the animals. The pinnate 

 tentacles are opaque white in colour, and have peculiar serpentine 



1 'Quart. Journ. Micr. Sci. xxxix. 1897, p. 32-3. 



