C. cylindracea, postice acuminata subfissa. [Plate XXV.] Fig. 36 and 37. Cylindric cercaria, the hind-part sharp and somewhat cloven.
It resembles the young ones of the podura[126] which live among the lemnæ, is pellucid, and appears to consist of a head, trunk, and tail; the head resembles that of a herring; the trunk is cylindric, replete with black spiral intestines, and appears more or less extended, at the will of the animal; nothing is to be discovered in the hinder-part. The tail most commonly appears to be divided into two bristles. The intestines are in a continual motion when the body moves, and by reason of their various shades give it a very rough appearance; some lateral hairs or cilia are likewise to be perceived. When it moves, it revolves at the same time as upon an axis. It is to be found in November and December, in marshy places that are covered with the lemna. Fig. 36, a, the head; b, the trunk; c, the tail; d, with one point; it is seen at e, Fig. 37, with two points; f, the hairs on the side.
[126] A genus of insects of the order of aptera. Linn. Syst. Nat. p. 1013.
130. Cercaria Viridis.
C. cylindracea mutabilis, postice accuminata fissa. Cylindrical cercaria, mutable, the lower end sharp, and divided into two parts.
This animalculum in some of its states considerably resembles the last, but has a much greater power of changing its shape. It is naturally cylindrical, the lower end sharp, and divided into two parts; but it sometimes contracts the head and tail so as to assume a spherical figure, at other times it projects outwards. It is found in the spring, in ditches of standing water.
131. Cercaria Setifera.
C. cylindracea, antice angustior, postice acuminata. Cylindric cercaria, the fore-part smallest, the hind-part pointed.
This is a small cercaria, the body rather opake, and of a round figure. The upper part is bright, and smaller than the rest; the trunk is more opake; the tail sharp, and near it a little row of short hairs. It has a slow rotatory motion. It is found in salt water, though but seldom.