139. Cercaria Pleuronectes.
C. orbicularis, cauda uniseta. Orbicular, the tail consisting of one bristle.
It is membranaceous, rather round, and white. In the fore-part are two blackish points; the hind-part is furnished with a slender sharp tail; it has orbicular intestines of different sizes in the middle; the largest of them are bright. Its motion is staggering or wavering; in swimming it keeps one edge of the lateral membrane upwards; the other folded down. It is found in water which has been kept for several months.
140. Cercaria Tripos.
C. subtriangularis, brachiis deflexis, cauda recta. [Plate XXV.] Fig. 38. Cercaria somewhat of a triangular form, two bent arms, and a straight tail.
The body is flat, pellucid, and triangular, having each angle of the base or fore-part bent down into two linear arms; the apex of the triangle is prolonged into a tail. It was found in salt water; b, the tail; a a, the bent arms.
141. Cercaria Cyclidium.
C. ovalis, postice subemarginata, cauda extersili. This is oval, the hind-part somewhat notched, with a tail that it thrusts out at pleasure.
It has an oval, smooth, membranaceous, and pellucid body, with a black margin. The tail is not fixed to the edge, but concealed under it, and comes out from it at every motion, but in such a manner, as to project but little from the edge. There is also a kind of border to the hinder-part. Its intestines are very pellucid vesicles. It is frequently found in pure water.