. A manual of zoology. Zoology. 30 GENERAL rRINCIPLES OF ZOOLOGY. -Ph iliclhys xifh iir. a, female (after Claus), X4; b, male (after Bergsoe), X13. The Nervous System.—This law applies as well to single organs as to entire animals. The central nervous system of many lower animals (eehinoderms, ccelenterates, many worms) forms part of the skin; in its first appearance it belongs to the surface of the body, because it has to meiiiate the relations with the external world. In the case of higher animals, , the vertebrates, the and spinal cord lie deeply imbed- ded in the interior of th
. A manual of zoology. Zoology. 30 GENERAL rRINCIPLES OF ZOOLOGY. -Ph iliclhys xifh iir. a, female (after Claus), X4; b, male (after Bergsoe), X13. The Nervous System.—This law applies as well to single organs as to entire animals. The central nervous system of many lower animals (eehinoderms, ccelenterates, many worms) forms part of the skin; in its first appearance it belongs to the surface of the body, because it has to meiiiate the relations with the external world. In the case of higher animals, , the vertebrates, the and spinal cord lie deeply imbed- ded in the interior of the body; but in the embr}'0 they are likewise laid down as a part of the skin (medullary plate) which gradually through infolding and cutting off from this comes to lie internally (fig. 9). T]ic Skelelal System.—The skeleton of vertebrates is a further example. In the lowest chordates, amphioxus and the cyclo- stomes, the vertebra; are lacking, and in their place we find a cylindrical cortl of tissue, the notochord. In the fishes and Amphibia the notochord usually persists; but it is partially reduced and constricted by the vertebnf, which in the lower forms consist of cartilage, and in the higher of bone. Mature birds and mammals finally have a com- pletely ossified vertebral column; their embryos, on the other hand, have in the early stages only the notochord (amphioxus stage); later this notochord becomes constricted by the vertebra? (fish-amphibian stage) and finally entirely replaced; the vertebral column is in the beginning cartilaginous, only later becoming ossified. Comparative anatomy and embryology thus give the same developmental stages of the axial skeleton: (i) notochord, (2) notochord and vertebral column, (3) vertebral column alone, the latter at first formed of cartilage, then of bone. We have spoken of a parallelism between the facts of comparative anatomy and of embryology. But we should expect a threefold parallel- ism; for according to the theoiy of evolu
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
