BALANO GL OSS US. 



247 



Far from assuming as a self-evident fact that the 

 extreme metamerism of the Annelids and Arthropods is 

 genetically identical with that of the Vertebrates, we have 

 every reason to suppose that it has been elaborated entirely 

 independently in the two cases, and that the apparent simi- 

 larity is due, as already intimated, to a pamllel evolution. 



Body-cavities ; Proboscis-pore ; Collar-pores. 



Corresponding to the 

 three regions into which 

 the body of Balanoglossus is 

 divided, - - namely, probos- 

 cis, collar, and trunk, --the 

 body-cavity is divided up into 

 three systems of cavities. 

 These are (a) the anterior 

 body-cavity or cavity of the 

 proboscis, (/3) a pair of collar- 

 cavities, and (j) a pair of 

 body-cavities which form the 

 unsegmented ccelom of the 

 trunk (Figs. 114, 115). 



These cavities arise essen- 

 tially as pouches from the 

 archenteron (Fig. 117), al- 

 though their actual develop- 

 ment differs considerably in 

 different species (MORGAN). 



The proboscis-cavity is 

 placed in communication 

 with the exterior by an open- 



mg 



through 



Fig. 116. Diagram of the organisa- 

 tion of Balanoglossus, from the dorsal 

 side. (From a drawing kindly lent by 

 Professor T. H. MORGAN.) 



c.p. Collar-pores, go. Gonads. g.s. 

 Gill-slits ; the dark lines converging be- 

 hind indicate the superficial portions of 

 the gill-slits; below the surface are seen 

 the free ends of the tongue-bars. //. 

 the posterior Proboscis-pore. Other letters as above. 



