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Updated: May 31, 2025
As Wilhelm Muller, of Jena, has shown, this rudimentary organ is the last relic of the hypobranchial groove, which we considered in a previous chapter, and which runs in the middle line of the gill-crate in the Ascidia and Amphioxus, and conveys food to the stomach.
We obtained, at the same time, large pieces of Labularia digitata, Sertularia abietina, upon which nothing of the animal kind was to be seen, but attached to which was frequently found Flustra dentata; also Pagurus Bernhardus, Fusus antiquus, Rostellaria pes pelecani, Cardium echinatum, Ascidia Prunum, Balanus sulcatus, Echinus saxatilis, and Spatangus flavescens.
This connation of the basal lobes is universally considered as a good and normal specific character. Nevertheless it has its manifest analogy in the realm of the anomalies. This is the pitcher or ascidium. It is probable that both these forms have varieties with, and others without, ascidia.
It has descended from the Cyclostoma by a profound degeneration, and these in turn from the fishes; even the Ascidia and the whole of the Tunicates are merely degenerate fishes! Following out this curious theory, Dohrn came to contest the general belief that the Coelenterata and Worms are "lower animals"; he even declared that the unicellular Protozoa were degenerate Coelenterata.
In the lower muscular wall of the gut we find a weal-like thickening, a solid, spindle-shaped string of cells, which becomes hollow in the centre; it begins to contract in different directions, now forward and now backward, as is the case with the adult Ascidia.
The whole of the anterior or respiratory section of the gut is converted into a gill-crate, which is pierced trellis-wise by numbers of branchial-holes, as in the ascidia. This is done by the foremost part of the gut-wall joining star-wise with the outer skin, and the formation of clefts at the point of connection, piercing the wall and leading into the gut from without.
Krause, on a convoluted body at the extremity of the tail in a Macacus and a cat. Kupffer, Prof., on the affinity of the Ascidia to the Vertebrata. Labidocera Darwinii, prehensile organs of the male. Labrus, splendid colours of the species of. Labrus mixtus, sexual differences in. Labrus pavo. Lacertilia, sexual differences of. Lafresnaye, M. de, on birds of paradise.
When the Ascidia-larva has attained this stage of development it begins to move about in the ovolemma. This causes the membrane to burst. The larva emerges from it, and swims about in the sea by means of its oar-like tail. These free-swimming larvae of the Ascidia have been known for a long time. They were first observed by Darwin during his voyage round the world in 1833.
It seems that the spores of crested leaves are more apt to reproduce the variety than those of normal leaves, or even of normal parts of the same leaves. But the experiments on which this assertion is made are old and should be repeated. Other cases of cleft leaves should also be tested. Ascidia are far more common than is usually believed.
It retains permanently the simple tubular form that we find temporarily as the first structure of the heart in the vertebrates. This simple heart of the Ascidia has, however, a remarkable peculiarity. It contracts in alternate directions.
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