Biological Papers. 127 



respiration, as thej^ do to-day; but both kinds of work were 

 poorly done, and part of the digestion must have been per- 

 formed, as now, in the entodermal cells, making digestion in 

 part cellular, as in modern protozoa. We know that recent 

 hydras and fresh-water sponges use algse, such as the protococ- 

 cus, to assist in the work of respiration. 



The whirlpool method of respiration in •.-^'- hes used in hy- 

 dras, polyps, snails, lampreys and hags muse have originated 

 in the Proterozoic era in sponges and hydras. Outlet for the 

 water must have been given then, as now, where it entered, or 

 through pores and slits at the bottoms of the pouches. 



But when the land habit was acquired, the drying influence 

 of the atmosphere made it necessary to give up using the ecto- 

 derm as a respiratory membrane, except incidentally, and to 

 make an increased use of saccules of the entoderm. Th« mam- 

 malian embryo still retains the instinct for making sacs, slit 

 so as to form gills ; but these generally close before birth, and 

 deeper-seated invaginations, many times sacculated, serve as 

 respiratory organs or lungs. 



The last organ-making instinct which my space will permit 

 me to mention has for its function the preparation of limbs 

 for locomotion. The earlier animals, so we learn from the 

 geologist, either spread themselves over the sea-bottom, or 

 were radially symmetrical with their axes in a vertical posi- 

 tion. The equilibrium of such animals was easily maintained, 

 but they were usually poor travelers. Worm-like animals were 

 possibly the first good swimmers. That these long animals 

 might not roll over and over, the body must have been flattened 

 dorsi-ventrally and the sides extended by lobe-evaginations, 

 -as we find in the Nereis of modern seas. Eventually certain 

 of these lobes must have become specialized as limbs for loco- 

 motion, as in centipedes, insects, fish, salamanders, and mam- 

 mals. 



As the mammals became better and better adapted to the 

 land habit of living, the ends of the subdivisions of these 

 evaginations became covered with hardened scales of the epi- 

 dermis. These eventually became the hoofs of the herbivorous 

 mammals, the claws of the carnivores, and the nails of man. 



As one studies the evolution of the body-building instincts 

 from perhaps a single primitive instinct that made its appear- 

 ance on earth fifty or sixty million years ago, as one learns 

 that these instincts grew and branched as the parts which 



