426 Causes and Course of Organic Evolution 



been changed into the stronger but more resisting double 

 "ehistica" or primary, and "fibrillar" or secondary sheath. 



As extruded by degrees from the sheath the nemertean 

 proboscis has laigely retained its former functional relations. 

 The inner non-eversible glandular part has separated as an 

 upgrowing pit that has fused with the infundibulum of the 

 brain to form the vertebrate pituitary body. So it has been 

 described as "a coecal diverticulum from the roof of the mouth" 

 (139: 370) while its continued secretion as a glandular struc- 

 ture in higher vertebrates is noteworthy. But its striking 

 union with the nasal sac in lampreys indicates how the pro- 

 boscis has distributed its tissue in evolving vertebrates. 



The middle highly muscular and glandular zone may have 

 originated on its dorsal side the uvula, while along its latero- 

 ventral sides the glands may be possible homologues of the 

 thymus or thyroids. The more muscular protrusible and 

 pa])illar portion might well suggest origin for the vertebrate 

 tongue wliich Jordan states to be extensible in Myxinidse, 

 wliile, as above indicated, the stylet or tooth-plate cells seem 

 to be the forerunners of the horny or calcareous accessory 

 teeth of vertebrates. 



The writer might here add, to the above very imperfect 

 comparisons, that he was guided to a study of the proboscis 

 and notochord relations several years ago, and only after he 

 had noted the many points of structural resemblance between 

 the groups concerned. These points will be taken up below. 



The surface of the body, in several genera or even groups 

 of Rotifera, e\ddently secretes a mucous substance, which 

 often acts in the cementing together of extraneous particles. 

 We do not as yet know whether this is due to excretions from 

 all of the cells, or whether from special gland-cells that are 

 proy)hetic of the mucus and other glands of rhabdocoels and 

 nemerteans. 



In Rotifera, the important external sense-centers are massed 

 in the cephalic area. So far as has yet been traced, these 

 centers are the circumoral disk, which from its mode of use, 

 and the general behavior of the animal when feeding, seems 

 to be a diffuse chemotactic center, which receives nerves from 

 the brain. No indication of an imdoubted olfactory organ 



