THE VERTEBRAE OF ARTICULATES 



61 



elementary pieces which unite round a central 

 space (Isis, loc. cit., p. 532). Serres had shown 

 that in the higher animals every vertebra is 

 formed from four centres of ossification, that 

 the body of the vertebra is at first tubular, 

 and that afterwards it becomes filled up. In 

 lobsters and crabs each segment is com- 

 posed of four elementary pieces, as may be 

 seen most easily in young ones. " Accord- 

 ingly each segment corresponds to a true 

 vertebra in composition : there is the same 

 number of ' materials,' the same order in 

 the course of ossification, the same kind of 

 articulation, the same annular arrangement, 

 the same empty space in the middle" (p. 

 534). The only difference is that in Articu- 

 lates the central space is very great and 

 contains all the organs of the body, whereas 

 in the higher Vertebrates the body of the 

 vertebra becomes completely filled up. In 

 the thoracic region of Crustacea it is not the 

 whole segment with part of the carapace 

 which corresponds to a vertebra, but merely 

 the part round the ventral nerve-cord (endo- 

 phragmal skeleton). 



If the skeleton of the segment in Articu- 

 lates corresponds to the body of a vertebra 

 and is here external, then the appendages 

 of the Articulate must correspond to ribs (p. 

 538). The full development of this thought 

 is found in a Memoir of 1822, " Sur la 

 vertebre." x He takes as the typical vertebra 

 that of a Pleuronectid, probably the turbot. 

 His original figure is reproduced (Fig. 2). 



He includes as part of the vertebra not 

 only the neural (e', e") and haemal (o', o") 

 arches, but also, above and below these, the 

 radialia (a", u') and the fin-rays (a', u"). 

 (Neither the radialia nor the fin-rays are, 

 1 Mem. Mus. d'Hist. nat., ix., pp. 89-119, Pis. v-vii. 



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