8 CHORD ATA. 



or head cavity ; it obviously corresponds to the proboscis cavity 

 and remains in the head region of the animaL The sacs of the 

 second pair are called the collar cavities, because they correspond 

 to those cavities of the Enteropneusta. They are in reality the 

 anterior pair of somites, and give rise dorsally to the first pair 

 of myotomes. Their exact disposition in the adult is not quite 

 certain, but they appear to get some backward extension. The 

 posterior sacs which come off as one pair from the enteron and 

 correspond to the trrnik cavities of Enteropneusta undergo in 

 subsequent growth a segmentation and give rise to the whole of 

 the mesoblastic somites of the trunk from the second pair back- 

 wards.* In the development and arrangement of its coelomic 

 sacs Amj)hioxus resembles in a remarkable manner the Entero- 

 pneusta, the difference between them consisting m the segment- 

 ation which the trunk cavities undergo in Amphioxus. 



In the Vertebrata, though it is not possible to point to such 

 close resemblances as those which we have just described, 

 there is a remarkable similarity in the embryonic arrangement. 

 The first coelomic sac is pre oral and unpaired ; the second is 

 paired and large, extending backwards in the mandibular arch, 

 so as to overlap the following somites. These mandibular 

 cavities are clearly homologous with the collar somites of the 

 other types. Following them we find on each side one large 

 cavity, the dorsal parts of which are divided up into segments and 

 become the myocoeles, and their walls the myotomes of the later 

 embryo. These posterior cavities clearly correspond to the trunk 

 cavities of the other types : as in them they are extensive, and 

 occupy the whole trunk region, and as in Amphioxus they are 

 metamerically segmented. 



In Amphioxus it is said that the preoral somite does not give rise to 

 striated muscles ; in Vertebrata it gives rise to a considerable number of 

 the eye muscles. 



With regard to the nonchordate phyla with enterocoelic 

 coelom, we have only space to say this, that in the Echinoderms 

 the Enteropneust plan of an unjoaired anterior cavity and two 

 pairs of posterior cavities can, according, to MacBride's researches, 

 generally be made out ; that in the Chaetognatha there is an ap- 

 proximation to the Enteropneust arrangement, but the unpaired 



* MacBride, Q.J. M.S., 40, 1898, p. 589. 



