Cope, "On the Phylogeny of the Vertebrata," Proc. Amer.
A similar constitution of the body is more clearly seen in the Chaetopod worms. In the Vertebrata also a repetition of units of structure (myotomes, vertebrae, &c.) - which is essentially of the same nature as the repetition in Arthropods and Chaetopods, but in many respects subject to peculiar developments - is observed.
Although the present article does not discuss mammalian osteology in general (for which see Vertebrata), it is interesting to notice in this connexion that the primitive condition of the mammalian tympanum apparently consisted merely of a small and incomplete bony ring, with, at most, an imperfect ventral wall to the tympanic cavity, and that a close approximation to this original condition still persists in the monotremes, especially Ornithorhynchus.
They are distributed thus: Vertebrata, 8 species; Insecta, 17; Arachnida, 12; Myriapoda, 2; Crustacea, 5; Vermes, 3; Mollusca, 1.
The nerve cells of the higher vertebrata, unlike their blood cells, their connective tissue cells, and even their muscle cells, early, and indeed in embryonic life, lose power of Nervous multiplication.