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The skeleton is a thickening of the horny cuticle of the annelid on the surface of each segment. The horny cylinder surrounding each segment is composed of several pieces, and on the abdomen these are united by flexible, infolded membranes. This allows the increase in the size of the segment corresponding to the varying size of the digestive and reproductive systems.
They are slender and jointed, and yet often very powerful. A large portion of the muscular system of the body is attached to these appendages. But the insect has also jaws. The annelid had teeth or claws attached to the proboscis. But true jaws are something quite different. They always develop by modifying some other organ. In the insect they are modified legs.
They were of use in swimming, and sections of them were stiffened with rods of cartilage, and became the pairs of fins. Some experts think, however, that the fish developed directly from a Crustacean, and hold that the Ostracoderms are the connecting link. Directly or indirectly, the fish is a descendant of some Archaean Annelid. It is most probable that the shark was the first true fish-type.
A little higher in the animal world a rude ear has developed, first as a very delicate organ for feeling the waves caused by approaching food or enemies; only later as an organ of hearing. Meanwhile the eye has been developing, to perceive the subtle ether vibrations. The eye of the turbellaria distinguishes only light from darkness, that of the annelid is a true visual organ.
There has been therefore in the successive forms and stages of animal life a clear sequence of dominant nervous actions. The actions of all animals below the annelid are mainly reflex or automatic, unconscious and involuntary. But in insects and lower vertebrates the highest actions at least are instinctive. Consciousness plays a continually more important part.
And it must already have become clear to you that the destiny of these different lines was fixed not so much directly by the skeleton itself as by its reflex effect in moulding the muscular, and ultimately the nervous, system. The insects formed their skeleton by thickening the horny cuticle of the annelid. They transformed the annelid parapodia into legs and developed wings.
These are regarded as surviving specimens of various groups of animals which, in early times, fell between the Invertebrate and Vertebrate worlds, and illustrate the transition. With their aid a genealogical tree was constructed for the fish. It was assumed that some Cambrian or Silurian Annelid obtained this stiffening rod of cartilage.
The excretory organs, kidneys, are composed primitively of nephridial tubes like those of the schematic worm or annelid, but immensely increased in number, modified, and improved in certain very important particulars. The muscles in simplest forms are composed of heavy longitudinal bands, especially developed toward the dorsal surface of the body to the right and left of the axial skeleton.
But the same may be said of every single arthropod and annelid if it be meant that all these organs are not contained in every possible slice. While if it be meant that parts of all such organs are contained in certain slices, then some of the mollusca may also be included. Another objection to Mr.
As yet, it must be admitted, the question is not decisively answered, several rival theories contending for supremacy in the case. One of the most important of these theories had its origin at the Naples laboratory; indeed, Dr. Dohrn himself is its author. This is the view that the type of the invertebrate ancestor is the annelid a form whose most familiar representative is the earth-worm.
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