AFFINITIES. 349 



Annelid Affinities. Dohrn, Semper, Beard, and others, maintain that 

 Annelids have affinities with Vertebrates. 



(i) Both Annelids and Vertebrates are segmented animals. 



(2) The segmental nephridia of Annelids correspond to the primitive 



kidney-tubes of a Vertebrate embryo. 



(3) The ventral nerve-cord of Annelids may be compared (in 



altered position) to the dorsal nerve-cord of Vertebrates. 

 Both cords are bilateral, and it is likely enough that the 

 tubular character of the spinal cord and brain is the necessary 

 result of its mode of development, and without much morpho- 

 logical importance. 



(4) Segmentally arranged ganglia about the appendages of some 



Chfetopod worms may correspond to the branchial and 

 lateral sense-organs of Ichthyopsida, and the ganglia asso- 

 ciated with some of the nerves from the brain. 



(5) The formation of the oral part of the pituitary body (see page 380) 



is suggestive of the way in which the mouth of Annelids is 

 sometimes formed. Perhaps the pituitary body represents an 

 old lost mouth and its ancient innervation. 



To minor points such as the red blood, well-developed body-cavity, 

 and slight internal skeleton of some Chsetopods, little importance can 

 be attached. 



The absence of anything like gill-slits in Annelids remains as a. 

 difficulty, even if we grant that no emphasis is to be laid on the tubular 

 nerve-cord of Vertebrates, and admit the possibility of an inversion 

 bringing the ventral nerve-cord to the dorsal surface. 



Nemertean affinities. Hubrecht and others have emphasised the 

 affinities between Nemerteans and Vertebrates. 

 In Nemerteans : — 



(1) The lateral nerve-cords sometimes approach one another ven- 



trally, and in rare cases dorsally. An approximation dorsal - 

 wards and union on that surface would result in a double 

 dorsal nerve-cord. 



(2) The firm dorsal sheath of the proboscis may correspond to a 



notochord. 



(3) The proboscis itself may correspond to the hyjjophysis or pitui- 



tary process characteristic of Vertebrate brains. 



(4) Two ciliated slits on the head may correspond to a pair of 



gill-clefts. 



(5) There is no segmentation, but the branches given off from 



the nerve-cords are sometimes serially arranged. 



It must be noted, that those who support those theories, 

 do not assert that any Nemertean or Annelid is in the direct 

 line of Vertebrate ascent. They simply emphasise the 

 demonstrable affinities. When these are thoroughly worked 

 out it will be possible to say what Invertebrate types are 

 most nearly related to Vertebrates. 



