A type of Planarian larva ([figs. 89] and [90])—possibly Plan. angulata, observed by Alex. Agassiz (No. [181]),—is very different from any other so far described, and is remarkable for being divided into a series of segments corresponding in number with the diverticula of the digestive cavity. In the youngest specimen ([fig. 89]) the body was nearly cylindrical, and divided into eleven rings, corresponding with as many digestive diverticula. Two eye-spots were present. In a later stage ([fig. 90]) the body was considerably flattened and had approached more to the planarian form.
If Agassiz’ interesting observations can be trusted we have in this larva indications of a distinct segmentation, which are of some morphological importance, especially when taken in connection with the traces of segmentation found amongst the Nemertines.
A further type, with an incomplete metamorphosis, has been observed by Girard (183). It is remarkable for having an uniform segmentation, and for presenting a quiescent stage after passing through a free larval condition with a large upper lip.
Fresh-water Dendrocœla. The development of the fresh-water Dendrocœla has been especially investigated by Knappert (No. [186]) and Metschnikoff (No. [188]).
The ova are very delicate minute naked cells, which to the number of 4‑6 or more become enveloped in a capsule or cocoon together with a large mass of yolk cells derived from the vitellarium. The yolk cells exhibit peristaltic movements and send out amœboid processes. Each ovum when laid becomes surrounded by an extremely delicate membrane, which disappears during the course of development. The capsules consist of a spherical case and a stalk. The latter is first emitted from the female opening as a thread-like body. Its free end becomes attached, and then the remainder of the capsule is ejected.
Impregnation takes place before the formation of the capsule. The segmentation is complete. The ovum first divides into two segments. One of these next divides, forming three segments. There are subsequently stages with four, eight, sixteen, and thirty-two segments.
Metschnikoff’s results on the stages subsequent to the segmentation are not in complete harmony with those of Knappert; but no doubt represent an advance in our knowledge, and I shall follow them here. His observations were made on Planaria polychroa.
In the earliest stage observed by him the segmentation was already far advanced, but no membrane was present round the ovum. At a later stage the ovum becomes more or less bell-shaped or hemispherical, and encloses within its concavity a mass of yolk elements. It is now formed of three concentric layers. An outer layer of flattened cells—the epiblast, a middle layer of fused cells—the mesoblast, and an inner solid mass of yolk cells—the hypoblast.
At the upper pole is formed the protrusible pharynx (cf. Knappert), which is provided with a provisional musculature and a lumen. By its contractions it takes up the yolk elements which surround the embryo, and the rapid growth of the embryo no doubt takes place at their expense. The embryo gradually loses its hemispherical form, and assumes an elongated and flattened shape. It acquires a coating of cilia by means of which it rotates. On the fifth day it is hatched.
The alimentary tract long remains solid, even after it has acquired its branched form. The pharynx becomes withdrawn as soon as the larva is hatched. It loses its provisional muscles, and subsequently acquires a permanent musculature. The young after hatching attach themselves to the body of their parent, on which they feed (?).