186 PROP. JOHN CLELANDj M.D., ETC., ON 



are only four primary divisions of the Vertebrata. Let us 

 glance at their relationship as regards typical character. 



In fishes there are seen all those characters which led 

 Carus to call the Vertebrata Cephalozoa, but so far from 

 the great regions of head and cervico-thorax and abdomen, 

 afterwards met with, being distinct in fishes they cannot 

 be said yet to exist ; the viscera are crowded forwards, 

 while the spinal cord, and muscular and osseous segments, 

 are produced backwards, till at last in one great modern group, 

 that to which the haddock and cod belong, not only is the 

 shoulder-girdle attached to the head, but the ventral fins 

 adhere to the shoulder-girdle and the cloaca is immediately 

 behind the ventral fins. 



But on leaving the Fishes, there is a more distinct 

 separation of regions having a peculiar relation to the whole 

 animal structure. The body-cavity remains devoted to the 

 structures of the vegetal sphere. The head contains the 

 highest and most characteristic organs of the animal sphere. 

 The neck is the original seat of development of the central 

 vascular system, though the heart is adventitiously pushed 

 in its later development into the part of the trunk which 

 becomes modified for its reception, namely, the thorax. 



The Vertebrata, higher than the Fishes, are groups deriving 

 their general facies and characteristic development from the 

 abdomen, the cervico-thorax and the head respectively. The 

 Cuvierian reptiles may be fairly regarded as abdominal 

 Vertebrata; the activity and domination of the circulatory 

 and respiratory system gives character to the Birds; the 

 development of the brain is the characteristic feature of 

 J\Iammals. 



I may be permitted to add what I pointed out to the 

 British Association at Exeter in 18G9, viz., that the limbs 

 are not developments of the individual segments of the 

 body, but belong each pair to a region, and that the 

 mandibular arch is the limb arch of the head, Avhile 

 the opercular bone of the fish, the stapes or columella of 

 other vertebrates, is the radiation or limb proper of that 

 arch. To prove this in detail would involve entering so 

 largely on the whole structure of the skull that I acknow- 

 ledge 1 have never had time to publish the proof in extenso. 

 I shall merely state that if this view is correct, then the 

 Vertebrata have three pairs of limbs, a pair for each region, 

 also that the limb of a region may be perfected in inverse 

 proportion to the central part. Thus the mandibular arch 



