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It is incontestable that the musculature of the Vertebrates has been evolved from that of lower Invertebrates; and among these we have to consider especially the unarticulated Vermalia. They have a simple cutaneous muscular layer, developing from the mesoderm.
Their lively nucleus divides quickly and often repeatedly, so that a number of new nuclei are formed in a short time; as each fresh nucleus surrounds itself with a mantle of protoplasm, it provides a new cell for the construction of the embryo. Their origin is still much disputed. Half of the twelve stems of the animal world have no blood-vessels. They make their first appearance in the Vermalia.
But the arrangement of these muscles and their relation to the solid skeleton are different in the Vertebrates from the Invertebrates. The human skeleton. From the right. The human skeleton. In most of the lower animals, especially the Platodes and Vermalia, we find that the muscles form a simple, thin layer of flesh immediately underneath the skin.
In these important Vermalia the foremost section of the gut has already been converted into a gill-crate, and the vascular arches that rise in the wall of this from the ventral to the dorsal vessel have become branchial vessels. We have a further important advance in the Tunicates, which we have recognised as the nearest blood-relatives of our early vertebrate ancestors.
The gonads are among the oldest organs, the few other organs that we find in the Platodes between the gut-wall and body-wall being later evolutionary products. They often have a number of branches. They are first met in the Turbellaria, and have been transmitted direct from these to the Vermalia, and from these to the higher stems.
Simultaneously with the construction of the medullary tube we have in the Amphioxus-embryo the formation of the chorda, the coelom-pouches, and the mesoderm proceeding from their wall. These processes also take place with characteristic simplicity and clearness, so that they are very instructive to compare with the vermalia on the one hand and with the higher vertebrates on the other.
We find the vascular system in this very simple form to-day in the Bryozoa, Rotatoria, Nematoda, and other lower Vermalia. The first step in the improvement of this primitive vascular system is the formation of larger canals or blood-conducting tubes. The spaces filled with blood, the relics of the primary body-cavity, receive a special wall.
It corresponds to the upper "gullet-ganglion" or "primitive brain" in other vermalia. Special sense-organs are either wanting altogether or are only found in a very rudimentary form, as simple optic spots and touch-corpuscles or tentacles that surround the mouth. The muscular system is very slightly and irregularly developed.
The same, showing the other organs. g brain, au eye, na olfactory pit, n nerves, h testicles, male symbol male aperture, female symbol female aperture, e ovary, f ciliated epiderm. Next to the ancient stem-group of the Turbellaria come a number of more recent chordonia ancestors, which we class with the Vermalia or Helminthes, the unarticulated worms.
In the first edition of this work I endeavoured to prove, on the strength of Kowalevsky's epoch-making discoveries, that "of all the animals known to us the Tunicates are undoubtedly the nearest blood-relatives of the Vertebrates; they are the most closely related to the Vermalia, from which the Vertebrates have been evolved.
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