Evolution of Animals 451 



the Nemertinea to the Cyclostomata and Batrachia. All 

 such evidence tends to show that the above is in all probability 

 the rapid and condensed line of evolution pursued from a 

 simpler invertebrate to a fairly high vertebrate ancestry. For 

 greater completeness, intermediate forms are specially desir- 

 able between the higher nemerteans and precyclostome forms. 

 But even though these may never be found in the fossil state 

 in virtue of their soft-bodied condition, and probably are not 

 now alive, we can fairly well picture the transition phases 

 that would be passed through. 



Reduced size of the proboscis, and shifting of its functions 

 to the steadily elaborating sense-centers and increasingly more 

 muscular head; resulting filling up of the sheath, as Hubrecht 

 has so far traced, by loose notochordal tissue; conversion of 

 the muscular w^alls of the latter into firmer but highly elastic 

 notochordal sheath tissue; added strength and pliability given 

 thereby to the body, which would result in increased growth 

 and vigor of movement; more perfect and rapid respiratory 

 interchange with resulting formation of gill pouches behind 

 the cervical furrows; increased concentration of the brain 

 lobes, and dorsal convergence of the lateral nerves to form 

 the spinal cord, as well as the specialization of the brain as 

 a sensory correlating center; these are a few of the modifi- 

 cations that must have been passed through as nemerteans 

 slowly became modified in some of their genera to a precy- 

 clostome condition. There seems no reason either to consider 

 that any of this took place other than in fresh-water or sub- 

 terrestrial surroundings. But in the process numerous off- 

 shoots may well have arisen that migrated seaward, and from 

 some of these we would derive the Hemichordata, the Uro- 

 chordata, and the Cephalochordata. 



An important histological modification that is often re- 

 garded as of craniate origin is the production of protective 

 or strengthening patches or plates of connective tissue and 

 of cartilage, as well as their later replacement by bone. One 

 or two closing suggestions on this may end the chapter. 



