Evolution of Animals 425 



becomes now the lining of the oral cavity, and as in some nemer- 

 teans retains the capacity to develop strong muscles and tactile 

 papillae, also glands for excretion of the juices into the oral 

 cavity. The mid region of the proboscis, the so-called tongue 

 in MyxinCy retains the capacity for development of pharyngeal 

 teeth, either a median homy tooth as in Myxine, or a series 

 spread over the entire area as in some nemerteans and in Petro- 

 myzon, or vomerine and other accessory buccal teeth as in 

 Batrachia. 



Further the sunken glands of this posterior buccal region, 

 where it joins on to the oesophagus in nemerteans, may well 

 represent the rudiments of the thyroid and of the thymus 

 glands of vertebrates. A critical and detailed study of these 

 in relation to vertebrate cephalic glands is highly desirable. 



That the notochord of vertebrates has somehow originated 

 as a structure that primarily had a different function has often 

 been expressed. Thus Wiedersheim (138: 34t) says: "The 

 notochoid or chorda dorsahs, lying in the longitudinal axis of 

 the embry^o between the neural and visceral tubes, is the first 

 of the endoskeleton to be formed, and is the forerunner of the 

 vertebral column. It is developed as a ridge of the hypoblast 

 from which it becomes constricted off, and is therefore of epi- 

 thelial origin [writer's italics]. The large parenchyma-like 

 cells of which it is composed consequently do not give rise to 

 any intercellular substance; vacuoles, however, soon appear 

 within the cells, the protoplasm of which undergoes modifica- 

 tion, and thus a retrogressive metamorphosis sets in. The 

 fact that this occurs at such early stages of development shows 

 that the notochord must long ago have begun to lose its primary 

 function^ whatever that function may have been' [writer's italics]. 



We can now sum up our studies of the relation of the pro- 

 boscis and its sheath in nemerteans as related to vertebrates 

 by saying that with Hubrecht we regard the sheath as a meso- 

 dermic but in all probability primarily an endodermally de- 

 rived tube for retraction of the proboscis. As to its present 

 origin from mesoblast or from invagination of the primary 

 epithelium, })oth such might well be adaptive possibilities. 

 By gradual filling up of the cavity from behind forward the 

 sheath has been converted into the notochord of vertebrates, 

 where it still retains the same loose cellular structure, investing 

 epithelium, and strong dou])le wall-sheath. The longitudinal 

 and transverse muscular coats of the sheath have gradually 



14* 



