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But the generic character of Zeugopterus, which is most important for the present discussion, is the prolongation of the dorsal and ventral fins on to the lower of the body at the base of the tail, the attachments of these accessory portions being transverse to the axis of the body.
As the same section of the ventral vessel, which also forms the heart in the Craniotes, has developed in the Ascidia into a simple tubular heart, we may regard the absence of this in the Amphioxus as a result of degeneration, a return in this case to the earlier form of the vascular system, as we find it in many of the worms.
The two small arrows indicate the entrance and exit of the water through the openings of the mantle. The hind end, which corresponds to the tail of the Amphioxus, is usually attached, often by means of regular roots. The dorsal and ventral sides differ a good deal internally, but frequently cannot be distinguished externally.
Nature yields to its suggestion and leading, and co- works, with all her best and busiest activities, to realise the human ideal; to put muscle there, to straighten that vertebra, to parallel more perfectly those dorsal and ventral lines, to lengthen or shorten those bones; to flesh the leg only to such a joint, and wool or unwool it below; to horn or unhorn the head, to blacken or blanch the face, to put on the whole body a new dress and make it and its remote posterity wear this new form and costume for evermore.
We may say, then, that the body consists of two tubes or cavities, separated by a bony wall, the dorsal or nervous tube, so called because it contains the central parts of the nervous system; and the visceral or ventral tube, as it contains the viscera, or general organs of the body, as the alimentary canal, the heart, the lungs, the sympathetic nervous system, and other organs.
Their lower position is shown also by the fact that the temperature of their bodies is, at least, ten degrees Fahrenheit below that of higher mammals. One of these carries the egg in a pouch on the ventral surface; the other, living largely in water, deposits its eggs in a nest in a burrow in the side of the bank of the stream. After these came the marsupials.
It is true that the Amphibia have two lungs, right and left. Ventral and pectoral walls are removed. The greater part of the body-cavity is taken up with the liver, from the middle part of which the caecum and the vermiform appendix protrude. Immediately behind the single structure of the thyroid gland a median groove, the rudiment of the trachea, is detached from the gullet.
It penetrates into the crab in its larval stage, and passes entirely into the crab's body, where it develops a system of branching root-like processes. When mature the body of the Sacculina containing its generative organs forms a projection at the base of the abdomen of the crab on its ventral surface, and after this is formed the crab does not moult.
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.
A mile or two from the centre of Rockland was a pretty little Episcopal church, with a roof like a wedge of cheese, a square tower, a stained window, and a trained rector, who read the service with such ventral depth of utterance and rrreduplication of the rrresonant letter, that his own mother would not have known him for her son, if the good woman had not ironed his surplice and put it on with her own hands.