One of the most remarkable of the Ctenophores is the “Venus’ girdle” (_Cestus veneris_), Plate V., Fig. 1. In shape it differs from all others of the class, as a comparison between it and the Beroë (Plate IV.), will show. It resembles in the daytime a silvery ribbon, or girdle, two or three feet in length, moving through the water by contractions of the body, rather than by the rows of combs that are found upon the edges. So delicate is this fragile creature, that it is almost impossible to remove it intact from the water. The mouth is in the centre, or equidistant between the ends; and on each side of it depends a short tentacle protruding from a sac. Opposite the mouth there is an otocyst, or sense-body. The combs, which are so conspicuous in other forms, are not so noticeable here, yet are well defined; and when moving along, and propelled by these gentle undulations, the _Cestus_ is one of the most beautiful objects of the sea. At night this wonderful sea-ribbon develops a new charm, emitting, according to Giglioli, a reddish yellow light of singular brilliancy. The Ctenophores, from their phosphorescence and great numbers, offer an interesting field for study. _Pleurobrachia_[9] may be found in myriads upon our eastern shores in the autumn. _Idya_[10] attracts immediate attention by its wondrous coloring, having a deep roseate hue. After death, its phosphorescence appears to be intensified, and much of the phosphorescent display is due to it. In nearly all the Ctenophores the light is erratic, flash succeeding flash, and seeming, according to Giglioli, to reside along the zone covered by the vibrating _cilia_, or little paddles. In the interesting group of animals known as _Physophoræ_,[11] or bubble-bearers, we find many light-givers of most remarkable form, in their structure reminding one of delicate objects in glass; and, according to Giglioli, all are more or less luminous. In the harbor of Gibraltar, he observed several beautiful forms, as _Abyla_, _Diphyes_, and _Eudoxia_; and in the Atlantic, in the latitude of Rio Janeiro, _Vogtia_, _Praia_ (Plate VI., Fig. 2), _Abyla_, and _Eudoxia_ were constantly encountered. These are all so fanciful in design, that they appear to be veritable fairy ships freighted with color-tints and gleams of light. Their luminosity is not scattered over the entire body as in many sea-jellies, but seems confined to fixed points, as in _Eucope_, a specimen of which, observed in the China Sea, seemed studded with brilliant emeralds, which appeared as marginal knobs at the base of the tentacles. In the Pacific, several species of _Diphyes_ have been observed, their zooids[12] brilliantly phosphorescent; but the hydroids of this group, so far as known, are not luminous.