CHAPTER IV 



THE COELOMIC OKGANS 



Introduction. — The mesoderm of Amphioxus consists in an early 

 stage, as already indicated (p. 57), of a row of closed sacs arranged 

 serially one behind the other upon each side of the body. At this 

 time the coelome of Amphioxus is in the extremely archaic condition 

 of a series of metamerically arranged paired compartments — a 

 condition resembling that of the less modified forms of Annelids. 

 The coelomic sacs gradually spread in a ventral direction until 

 they meet. For a time after this happens the sacs of opposite sides 

 of the body remain separated by a longitudinal partition the ventral 

 mesentery. Similarly the apposed posterior and anterior walls of 

 neighbouring sacs belonging to the same side of the body, form 

 thin membranous septa like those of Annelids. 



A highly characteristic difference from the Annelid arrangement 

 begins to show itself a little before hatching in the ventral portion 

 of the body, in as much as the transverse septa break down and 

 disappear thus converting what was hitherto a chambered coelome 

 in this region into a continuous space. There is no obvious reason 

 why this loss of segmentation of the ventral portion of the meso- 

 derm has come about in evolution. A general characteristic, 

 however, of the phylum Vertebrata is the loading up of the ventral 

 part of the endoderm with yolk and it may well have been that the 

 loss of the mesoderm septa ventrally arose in correlation with the 

 presence of a greater amount of yolk in the ancestral condition than 

 exists in the present-day Amphioxus. 



A further striking difference between the Vertebrate and the 

 Annelid is expressed in the extent to which the coelomic wall gives 

 rise to muscular tissue. In the Annelid practically the whole extent 

 both of the somatic layer lining the body -wall and the splanchnic 

 layer covering the gut gives rise to muscular tissue. In Amphioxus 

 however, and the same holds for Vertebrates in general, the ventral 

 portion of the somatic mesoderm, the portion which loses its segmental 

 character — loses also its capacity for producing muscle. 



On the other hand the dorsal portion of the mesoderm, which 

 retains its segmentation, retains also* and to an accentuated degree, 

 its muscle-forming capacity. It separates off from the ventral or 



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