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Show LECTUI{E IV. ON THI£ ULASSI.FICATIO~ OF ANll\1ALN. 'l'HE VERTEBR.Arl'A; R PISCES, AMPHIBIA, REP'l'ILIA, AVES, AN u MAMMALIA. IN the rapid snrvoy of the animal ]~ingdom with which we have been occupied in the preceding lectur , I have, for roasm1s which will be obvious by and by, taken group by group, and considered each separately upon its own merit ~, without attnmpting to say anything of the chara ·teristic of tho larger divisions into which these classes may be arrang d. That is a point to which I shall return on a future occa ion. But with those animals 'vbi h arc call d "vertebrated," such a course as this would involve a, great and unprofitable expenditure of time and much repetition; b cause the five groups of animals which pass under ihis na1ne-the c]a, os Pisces, Amphibia, Reptilia, Aves, and 1Jfa1n1nalia-ar bviously united and bound together by many com1non charact ristics, and are well known to be so connected. I shall con1m nee the present lecture, therefore, by ennm rating the mo t i1nportant of those structural peculiarities 'vhich thes five great clivi ions exhibjt 111 common. In the animals to which our attention has hith rto been confined, the external, or integumentary and pari tal, portion of the blastoderm never b come. developed into n1ore than a Hingle saccular, or tubular, investm nt, which inclof:!es all the viscera. So that if we make a transverse section of any one of these anilnals endowed with a .·nfficiently high organization 'l'FH~ VJ£B.TEBHA'l'A. 5U to po~~es~ a, nervous ~ystem awl a heart, that seetion 1nay L) represented diagrmnmatically as in Fig. 30 (I.), whore P repre- Fig. :-w . . II. M Fig. 30.-Dingrams representing generalised .sedious of one of the higher l!tvct:teurat.L•s (I. If. ), and of a Vertebrate (III. IV.); I. III. transverse, II. IV. longJtudlllal. ::;t'etion. A, alimentary canal; II, heart; P, parietes of the body; P', par1 tl'S of the neural canal; N, nervous centres of Invertebrate; Nl, sympathetic, mHl N2, cerebro· spinal centres of Vertebrate; ch, notochord; M, mouth. sents the parietes or wall of the body, A the alimentary canal, ll the heart, and N the nervous centres. It will be observed that the alimentary canal is in the middle, the principal centres of the nervous system upon one side of it, and the heart upon tho other. In none of these animals, again, would you discover, in the embryonic state, any partition, formed by the original external parietes of the body, between the nervous centres and tho alimentary canal. But, in the five vertebrate classes, tho parietal portion of tlw blastoderm of the embryo always becomes raised up, upon each side of the middle line, into a ridge, so that a long groove is formed between the parallel ridges thus developed ; and the margins of these, eventually uniting with one another, constitute a second tube parallel with the first, by a modi.ficatiou of the inner walls of which the vertebrate cerebro-spinal nervous centres are developed. Hence it follows that, after any vertebrated animal has passed through the very earliest stages of its development, it is not u, single, Lut a double tube, and tho two |
