THE STKITOTUKE AND AFFINITIES OF SACCOOIRRUS. 423 



primarily covered by the ccelomic epithelium — even in the 

 adult ; and in Polygordius thenephridia are not always in the 

 body-wall, as stated by Fraipont, but may run (as in Poly- 

 ophthalmnus, for instance) along the oblique muscles crossing 

 the ccelom, as shown in fig. 7. The persistence of both 

 mesenteries is probably a truly archaic character. This and 

 the absence of the parapodia and chsetas are the only points 

 of first-rate importance common to Polygordius and Proto- 

 drilus in the list of alleged primitive characters. It is^ then, 

 to the absence of parapodia and cheetas that we must turn 

 our attention. 



Let us try, for the sake of argument, to conceive what sort 

 of creature the common ancestor of the Annelida (Polychgeta, 

 Oligochseta, Hirudinea, Echiuroidea, and Myzostomaria) 

 must have been. It may safely be conjectured that it was a 

 creeping segmented worm, with a skin, whether ciliated or 

 not, covered by a cuticle ; with metameric bundles of cheetee, 

 longitudinal ventral nerve-cords ; circular, longitudinal, and 

 dorso-ventral muscles, septa, and mesenteries ; with seg- 

 mental nephridia (probably not opening into the coelom), 

 ccelomic cavities and coelomostomes (genital ducts) leading 

 to the exterior. Such may have been approximately the 

 structure of the Annelid common ancestor in pre-Cambrian 

 times. This primitive Annelid must have been itself derived 

 from a form which, we may presume, to some extent ap- 

 proximated to the plan of structure now elaborated along 

 the Nemertine and Platyhelminth lines of descent. In other 

 words, it was probably derived from worms in which the 

 muscles and parenchyma were well developed, but in which 

 the coelom and internal segmentation were not so well differ- 

 entiated, and which were provided with nephridia ending in 

 flame-cells. 



Now this is just the position the Polygordiidae occupy 

 according to the Archi-annelid theory. Can it be truly said 

 that they fit in the place assigned to them ? Moreover can 

 it be believed that these little modified ancestral forms have 

 persisted to the present day, and live happily together with 



