Origin of the Vertebrates. 255 



in the Annelid Worms is an objection that may be lodged 

 against the annelidan theory of the Vertebrates. It is easily 

 disposed of. The branchial clefts are, in fact, but adven- 

 titious orifices of lateral diverticula of the alimentary canals. 

 Now many Annelid Worms exhibit such diverticula ; they 

 are also produced in the Turbellaria and the Nudibranch 

 Mollusks, which are nephridiates, and in this case these diver- 

 ticula may open to the exterior {Yungia, Cgcloporus, 



Balanoglossus itself, were it tlie ancestor of the Vertebrates, 

 would come to be intercalated between them and the Annelid 

 Worms, and would bear witness to the possibility of the 

 appearance of pharyngeal clefts in these latter. The pheno- 

 mena of budding in various CTEXODRiLiDiE, SyllidtE, and 

 Serpulix^, as well as in the Natdomorpha, also demon- 

 strate the possibility of the appearance of adventitious orifices 

 to the digestive canal in the case of the Annelid Worms 

 properly so-called. 



(4) Circulatory Apparatus. — Among the ancestors attri- 

 buted to the Vertebrates, the Annelid Worms, the Nemer- 

 tines, and BaJannglossus have, like the Vertebrates them- 

 Kelves, a closed circulatory apparatus ; on the other hand, no 

 Arthropod is known the circulatory apparatus of which is not 

 in part lacunar. On this head the Arthropods are therefore 

 excluded for the second time. The circulatory apparatus of 

 the Nemertines has no differentiated propulsive centre ; that 

 which is called the heart in Balanoglossus is an organ situated 

 on the same side of tlie alimentary canal as the nervous axis, 

 and cannot consequently be homologous with the heart of 

 the Vertebrates ; the Annelid Worms tiierefore alone exhibit 

 the typical relations of the circulatory centres, alimentary 

 canal, and nervous axis, which are observed in the case of the 

 Vertebrates. 



(5) Nephridial Apj^arafus. — The urinary apparatus of the 

 true Arthropods* is constructed after quite a different type from 

 that of the uephridial apparatus of tiie Annelid Worms, the 

 Nemertines, Anqihioxus, and the Vertebrates; this apparatus 

 appears to be wanting in Balanoglossus, \y\nc\\ thus furnishes 

 a new motive for its exclusion ; in the case of the Annelid 

 Worms and in Amphioxus it is formed of independent ciliated 

 tubes, opening into the general cavity on the one side and to 

 the exterior on the otiier, and recurring regularly either in 

 almost all the segments of the body or only in a certain 



* Among these Peripatus cannot be included with certainty, in spite 

 of the attempts that have recently been made to connect it with them. 



