ON THE DIPI.OCHORDA. 331 



4. The vascular system is primitive, cousistiug, when present 

 at all, of sinuses and splits between the coelomic walls. 



5. The protomere (pre-oral lobe) is usually present through- 

 out life, or is distinctly in evidence at one time in the ontogeny 

 of the individual. 



6. The ccelom is segmented into a protocoele in the pre-oral 

 lobe, a pair of mesocoeles, and a metacoele which tends to be 

 secondarily more or less divided up into two. Each coelomic 

 cavity has primitively a pore leading to the exterior, which 

 primarily acts as a gonaduct and nephridiopore. The proto- 

 coelic pore is divided usually into two proboscis pores, the 

 mesocoelic pores are usually termed collar pores ; all these 

 early lose their gonaducal function. The raetacoelic pores 

 usually become specialised for gonaducts. 



These will indicate a few characters of the Archicoelomata, 

 and it will be seen that the Archichorda (Cephalo- 

 discus, Phoronis, and Balanoglossus), Echinodermata, 

 Chaetognatha, and Brachiopoda, all readily group them- 

 selves under this head, and there can be little doubt that the 

 Sipunculoidea and Polyzoa will be found to conform to 

 the same character. 



As might be expected of animals preserving a primitive 

 type of structure, nearly the whole of these groups are essen- 

 tially of sedentary or burrowing habits, though the Chaetog- 

 natha are a remarkable exception. The position of the anus 

 and the retention of the reproductive function in the meso- 

 coeles in this group indicate an early assumption of the group- 

 characters, and the very existence of this group shows that, 

 under certain conditions,^ the archicoelomate type has been 

 able to maintain its existence in competition with metameri- 

 cally segmented forms. Amongst these Archicoelomata 

 we may notice one group, the Archichorda, which are nearest 

 the direct line of the true metamerically segmented Chordata; 

 and others again, such as the Sipunculoidea and Brachio- 

 poda, which show, in their ontogeny, stages in the evolution 



* The Chaetognatha are probably a primitively pelagic group. 



