1887.] NATURAL SCIENCES OF PHILADELPHIA. 349 



has occurred, giving rise to the gutless and acoelomatous tail of a 

 number of classes, the shortening has been compensated for by the 

 development of intestinal coils, diverticula, pouches, rugae, folds, 

 follicles, crypts, glands, valves, etc., thus increasing the efficiency 

 of the anterior portion of the alimentary canal as a digestive appa- 

 ratus, so that the hinder part became useless, thus leading to its 

 suppression. This seems to have been the cause antecedent to the 

 one invoked by Dohrn. 



Accepting the morphological part of the theory of Dohrn, as to 

 the phylogeny and ontogeny of the paired and unpaired limbs, a 

 difficulty has arisen as to how the parapodia became mainly dorsal 

 and ventral, if, as is supposed, they are derived from an ancestry in 

 which they were lateral. The view that the presence or absence of 

 the alimentary canal was an efficient cause is beset with grave diffi- 

 culties. A view which seems to me to be far more probable, rests 

 upon the exaggerated development of a very different set of paired 

 structures in the lower vertebrates, namely, the myotomes. The 

 homologous tract in the Annelids is almost equally developed around 

 almost the entire circumference of the somites, whereas in the ver- 

 tebrates their development is almost wholly lateral, especially dur- 

 ing the early stages. The bilaterality of the muscular system thus 

 finding expression in the much thickened or laterally hypertrophied 

 somatopleure of the primitive vertebrji tes, would inevitably crowd 

 the notopodia and neuropodia of the ancestral Annelid, toward the 

 ventral and dorsal edges of the body ; the infolding of the medull- 

 ary groove would divide the somatopleure in the middle line, and 

 the growth forward of the stout notochord would tend greatly to aid 

 from beneath, in breaking the continuity of the somatopleural layer 

 across the median line. The suppression of the dorsal moieties of 

 the body cavities in the myotomes would also aid in effecting the 

 needed change. 



With the advent of a laterally hypertrophied somatopleure, flex- 

 ures of the body in the ventral or dorsal, as well as lateral direction 

 would no longer be so well marked, and the habitual flexures of the 

 body now established, would be alternating ones from right to left ; 

 thus, the habitual mode of flexure of the body of fishes would be at- 

 tained. The mode in which muscular contractions resulting in the 

 manifestation of movements would thus become more specialized 

 than in the annelids and be brought to the stage observed in the 

 lower vertebrata. 



