The type most fully investigated by Kowalevsky is Ascidia (Phallusia) mammillata; and the following description must be taken as more especially applying to this type.
On our two plates, theascidia is marked by A, the amphioxus by B.
In the larva of the ascidia this rod (c) proceeds along the long rudder-tail, a larval organ, which is cast off in later transformation.
The early stages of the Ascidiapossess the beginnings of the spinal marrow and the spinal column (chorda dorsalis) lying beneath it, which are the two most essential and most characteristic organs of the vertebrate animal.
The production of ascidia or pitchers from the cohesion of the margins of one or more leaves has been already alluded to (see pp.
Kovalevsky's discovery of the close identities of the development of organs in Ascidia and Amphioxus.
The latter affirmed that the nervous system of Ascidia originated from the upper layer, whilst Elie believed that it was the lower layer which gave birth to it.
In 1868 Metchnikoff had a dispute with Kovalevsky as to the origin of the nervous system of Ascidia (p.
In Ascidia canina these processes become as long as the whole diameter of the vitellus (Kupffer, No.
It is particularly interesting and instructive in this connection to compare the Enteropneusts with the Ascidia and the Amphioxus (Figures 2.
We meet the same simple medullary tube in the first structure of the ascidia larva, in the same characteristic position, above the chorda.
They take place in the same way as we find them in many of the lower animals of different invertebrate stems, which we have already described as original or primordial; the development of the Ascidia is of the same type.
There is no trace in the fully-developed Ascidia of a chorda dorsalis, or internal axial skeleton.
The food of the Ascidia also consists of tiny organisms, infusoria, diatoms, parts of decomposed marine plants and animals; etc.
Thus the embryology of the Amphioxus and the Ascidia has so much increased our knowledge of man's stem-history that, although our empirical information is still very incomplete, there is now no defect of any great consequence in it.
In the adultAscidia the branchial gut and the heart on its ventral side are almost the only organs that recall the original affinity with the vertebrates.
In the Ascidia (as in the other case) the oval gastrula is first flattened on one side--the subsequent dorsal side.
These free-swimming larvae of the Ascidia have been known for a long time.
If we now glance at the entire structure of the simple Ascidia (especially Phallusia, Cynthia, etc.
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!
The branchial or head-gut of the Ascidia is small at first, and opens directly outwards only by a couple of lateral ducts or gill-clefts--a permanent arrangement in the Copelata.
Found in the branchial sac of Ascidia mentula and communis.
Some species have more rounded and some more elongated ascidia and often the shape is seen to change with the development of the stem.
It had produced seven ascidia in all, each formed by the conversion of one leaflet on the trifoliolate leaves.
In this way it is easily conceivable that peltate leaves are a frequent specific character, whileascidia are not, as they only appear in the special cases of limited adaptation, as in the instances of the so called pitcher-plants.
Those of the Croton, according to our knowledge regarding similar cases, must [674] have arisen at once, and hence the conclusion that the ascidia of Nepenthes are also originally due to a sudden mutation.
In the same way doubleascidia may be made use of to explain the foliar cups of the teasels and some other plants, as for instance, some European snakeroots (Eryngium maritimum and E.
I have seen many hundred ascidia on one lime-tree, and far above a hundred on the magnolia.
The reproduction of specific characters by anomalous ascidia is not at all limited to the general case as described above.
All in all, the tendency to produce ascidia increased from the beginning to the tenth leaf, and decreased from this upward.
Ascidia are far more common than is usually believed.
Ascidia of the white clover have been found in numbers, in my own experiment-garden, but always in the springtime.
As far as we can judge peltate anomalies are quite uninjurious, while ascidia are forms which must impede the effect of the light on the leaf, as they conceal quite an important part of the upper surface.
It should be added that this explanation of the different shapes of ascidia is of general validity.
Ascidia afford another instance of this periodicity, though ordinarily they are by far too rare to show any regularity in their distribution.
Rare instances point [428] to poor races, but the magnolias and lime-trees are often so productive of ascidia as to suggest the idea of ever-sporting varieties.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "ascidia" in a variety of sentences. We hope that you will now be able to make sentences using this word.
