Origin of the Segmented Worms. 519 



that suggested by Professor Sedgwick, that they were at one 

 time due to foldings of the digestive layer — is the reproductive. 

 But I cannot help remarking that the view here put forward, 

 which regards the mesoderm as arising by gradual structural 

 modifications in response to the simplest and most frequently 

 recurring of life's activities, viz. to locomotion in pursuit of 

 food — that is, to locomotion which lasts from the beginning 

 to the end of life, — supplies us with a better motive for its 

 production than do the needs of the reproductive function, 

 which are periodic and confined to the adult. Further, I 

 need hardly again emphasize the fact that the suggested 

 origin put forward in this paper avoids all appeals to " change 

 of function " ; from its earliest rudiments to the highest 

 specialization the mesoderm, if developed along the lines here 

 sketched, had essentially the same function. 



With the perfection of the powerful neuro-muscular sheath 

 and the necessary mesodermal apparatus for its nutrition and 

 excretion we have brought our strings of Coelenterate buds 

 near enough to the typical Annelid to show that, whether 

 this was the true origin of the Annelids or not, it could easily 

 have been so. It is admittedly only an outline sketch. A 

 much closer comparative study of kindred animals and of 

 their homologous structures would be necessary in order to 

 fill in details. One somewhat prominent point may, how- 

 ever, be noted. 



For the purpose of bringing about the strong lateral con- 

 tractions of each segment necessary to give the whole complex 

 its serpentine motion, the longitudinal muscles would tend to 

 stretch from end to end of each segment. Their points of 

 insertion would consequently be more and more concentrated 

 at the ends of the buds, leaving the middle regions free. As 

 already noted, the ectoderm was the only available respiratory 

 surface. What more natural than that this middle region 

 should become specialized for respiration, the greatly developed 

 neuro-muscular system demanding for its efficiency ever- 

 increasing supplies of oxygen. The necessary mechanism 

 for obtaining this extra supply was well within reach and the 

 structures developed would be from the first functional. I 

 refer to the lateral skin-folds which would arise in the middle 

 regions above mentioned every time the longitudinal muscles 

 contracted. From this point of view the parapodia, whatever 

 specialization and differentiation they may since have under- 

 gone, were originally nothing more than gill-folds, and they 

 arose concomitantly with the rise and concentration of the 

 powerful longitudinal muscles as these latter required more 

 and more oxygen for their work. 



