BALAArOGLOSSUS. 



247 



Far from assuming as a self-evident fact that the 

 extreme metamerism of the AnneHds 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 parallel 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, (;S) a pair of collar- 

 cavities, and (7) a pair of 

 body-cavities which form the 

 unsegmcnted ccelom of the 

 trunk (Figs. 114, 1 15). 



These cavities arise essen- 

 tially as pouches from the 

 archenteron (Fig. 117), al- 

 though their actual develop- 



. ^- re -1 11- FiK. 116. — Diarram of the or^anisa- 



mcnt differs considerably m „„„ ^^ Baianogiossus, from the dcsai 



different species (Morgan), side. (From a drawing kindly lent by 



Professor T. H. MORGAN.) 

 The proboscis-cavity is c.p. Collar-pores. go. Gonads. g.s. 



11- ■ .■ Gill-slits: the dark linos converginEr be- 



placed in communication ^^^j j^^'^^^,^ „^^ superficial portions of 



with the exterior by an open- "i« gill-sHts; bcIow the surface arc seen 



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

 ing through the posterior l>roboscis-porc. Other letters as above. 



