THE BODY CAVITY. 



627 



suggested that the abdominal pores are perhaps remnants of the openings 

 of segmental tubes ; there does not however appear to be any definite 

 evidence in favour of this view, and it is more probable that they may have 

 arisen as simple perforations of the body wall. 



Pericardial cavity, pleural cavities, and diaphragm. 



In all Vertebrata the heart is at first 

 placed in the body cavity (fig. 353 A), 

 but the part of the body cavity contain- 

 ing it afterwards becomes separated as 

 a distinct cavity known as the pericar- 

 dial cavity. In Elasmobranchii, Aci- 

 penser, etc. a passage is however left 

 between the pericardial cavity and the 

 body cavity ; and in the Lamprey a 

 separation between the two cavities does 

 not occur during the Ammoccete stage. 

 In Elasmobranchii the pericardial 

 cavity becomes established as a distinct 

 space in front of the body cavity in the 

 following way. When the two ductus 

 Cuvieri, leading transversely from the 

 sinus venosus to the cardinal veins, be- 

 come developed, a horizontal septum, 

 shewn on the right side in fig. 352, is 

 formed to support them, stretching 

 across from the splanchnic to the so- 

 matic side of the body cavity, and 

 dividing the body cavity (fig. 352) in 

 this part into (i) a dorsal section formed 

 of a right and left division constituting 



FIG. 



-ht 



SECTION 



THROUGH THE TRUNK OF A 

 SCYLLIUM EMBRYO SLIGHT- 

 LY YOUNGER THAN 18 F. 



The figure shews the sepa- 

 ration of the body cavity from 

 the pericardial cavity by a 

 horizontal septum in which 

 runs the ductus Cuvieri ; on 

 the left side is seen the narrow 

 passage which remains con- 

 necting the two cavities. 



sp.c. spinal canal ; w. white 

 matter of spinal cord ; ft: 

 commissure connecting the 

 posterior nerve-roots; ch. noto- 

 chord ; x. sub-notochordal 



heart ; pp. body cavity ; pc. 



pericardial cavity; a~s. solid 



oesophagus ; /. liver ; m p. mus- 

 cle-plate. 



the true body cavity (/>/>) and (2) a 

 ventral part the pericardial cavity (fc). 

 The septum is at first of a very small 



. . .. . , , , 



longitudinal extent, so that both in 



front and behind it (fig. 352 on the left 



side) the dorsal and ventral sections of the body cavity are in 



free communication. The septum soon however becomes pro- 



longed, and ceasing to be quite horizontal, is directed obliquely 



upwards and forwards till it meets the dorsal wall of the body 



40 2 



