PHYLOGENY 



The Annelid Theory. With greater reason, it has been suggested that 

 the annelids may have been the source of the chordates. This is based 

 upon the resemblance of primitive chordates, such as the Ammocoetes 

 larva of the lamprey, to an inverted annelid. This may best be visualized 

 by studying Figure 51, first upright, then with the book inverted. The 

 digestive system in either case is a simple tube with a ventral mouth at 

 one end and an anus at the other. The nervous system of the annelid con- 

 sists of a pair of ventral cords and cerebral ganglia which are connected 

 to the ventral cords by a pair of commissures which form a ring around 

 the esophagus. If the worm were to form a chordate by turning over on 

 its back, it would be necessary for the original mouth to close and a new 

 one to form on the new ventral (former dorsal) surface. The nervous 

 system would then be entirely dorsal, with the cerebral ganglia forming 

 the brain. The circulatory system of the annelid is based upon a dorsal 

 vessel in which the blood flows anteriorly and a ventral vessel in which 

 the blood flows posteriorly. At the anterior end, there is a series of pul- 

 sating vessels in which the blood flows from the dorsal vessel to the ven- 

 tral. If inverted, this would approximate the primitive chordate system 

 in which a dorsal aorta carries the blood posteriorly, the posterior cardinal 

 veins and ventral aorta carry it anteriorly, and the aortic arches connect 

 the ventral and dorsal vessels at the anterior end. Further, the annelids 

 and the chordates are the two most conspicuously segmented phyla. 



The resemblance is thus extensive, but there are some serious difficul- 

 ties. First, there is the difficulty of forming a new mouth. Then there is no 

 structure in any annelid which even remotely suggests a notochord. And 

 although the "hearts" which connect the dorsal and ventral vessels might 

 be homologized with the aortic arches, there is no suggestion of gill slits 

 in the annelids. Further, the annelids show the typical protostome embry- 

 ological characters, while the primitive vertebrates show the typical deu- 



ANNELID 



Figure 51. Diagram to Illustrate the Supposed Transformation of an Annelid 

 Worm into a Vertebrate. In normal position this represents the annelid with a 

 "brain" (s) at the front end and a nerve cord (x) running along the underside of the 

 body. The mouth ( /n ) is on the underside of the animal, the anus ( a ) at the end of 

 the tail; the blood stream (indicated by arrows) flows forward on the upper side of the 

 body, back on the underside. Turn the book upside down 



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