518 Mr. II. M. Bernard on the 



far stronger tlian any which the bud itself would require had 

 it existed as a single animal. Similarly the nervoits system 

 necessary to coordinate the movements of these muscles 

 within each bud and with those of the buds before and behind it 

 would have to be very much more higiily developed than would 

 have been necessary in single animals the size of the buds or 

 segments. We need not be surprised, then, at the enormous 

 development of the mesodermal chambers. It is significant 

 here to note that the Turbellarians, which I am inclined to 

 regard as the nearest existing kindred of our early strings of 

 buds, having, with a few exceptions, lost the power of forming 

 buds, have their muscles and nerves simply embedded in a 

 parenchymatous tissue not unlike that which we imagine to 

 have been a stage in the development of the mesodermal 

 cavities of the Annelids. Neither muscles nor nerves in 

 these single animals required the same high specialization as 

 was required by the individuals forming the separate links 

 in a long chain which had to move as one, comparatively 

 speaking, immense composite organism. We thus have, in 

 the specially pronounced development of tlie neuro-muscular 

 sheath of each segment of our string of buds — a far more deve- 

 loped sheath, I repeat, tlian any single animal could possibly 

 require, — an efficient cause to account for so highly specialized 

 a nutritive and excretory apparatus as the mesoderm of the 

 Annelids, with its cavities, vascular system, and nephridial 

 funnels. 



On turning to the embryological records of the mesoderm 

 we find them confused, and, for want of a clue, frequently 

 contradictory. We have the coelomic cavities arising in 

 masses of parenchymatous tissue, or forming early out of very 

 few cells, or^ again, as definite invaginations. All these fall 

 into their places in the light of the origin above assigned to 

 the mesoderm. They are fragments and abbreviations of the 

 true phylogenetic history. The invaginations may be either 

 for the purpose of rai)idly supplying cells or else as shortened 

 methods of forming the coelomic cavities. Tliey obviously 

 serve both purposes. 



We have here an excellent illustration of the difficulty 

 of discovering phylogeny from embryology. The parenchy- 

 matous mesoderm suggests little or nothing, whereas meso- 

 dermal invaginations seem so clearly to indicate some once 

 useful structures which, by " change of function," might 

 have supplied the mesoderm, that they immediately acquire 

 a greater importance than really belongs to them. One 

 primitive function to which they have been attributed — besides 



