528 HISTORY OF THE HUMAN BODY 



pharynx in the form of a ventral groove. This is the endo- 

 style, an organ that performs a very important service in the 

 collection and retention of food. Its surface is covered with 

 cilia, which produce a backward directed current, and it is 

 furnished also with gland cells that secrete a viscous fluid. 

 The nutrient particles that enter the pharynx in the water 

 current are engaged by the viscous fluid, and the combined 

 mass is conveyed to the intestine by the motion of the cilia. 



The main blood-vessels consist of a sub-intestinal vein, ven- 

 tral to the alimentary canal, in which the current is directed 

 forwards, and an aorta, situated between the intestine and the 

 notochord, in which the blood flows posteriorly. Aside from 

 these there is a series of branchial vessels, a pulsating heart, 

 and other vessels, the relations of which are like that found in 

 vertebrates, but simpler. 



The general impression given of the structure of Amphioxus 

 is that of a diagrammatic vertebrate modified by several sec- 

 ondary adaptations. Had it been known to Goethe, he would 

 have almost denominated it the realization of the primordial 

 vertebrate, an incarnate " Urbild." In the general arrange- 

 ment of the essential organs, the dorsal nervous system in 

 tubular form, the notochord, the dorsal aorta, the intestine 

 with its pharynx perforated by gill-slits, it is essentially verte- 

 brate; while in its peribranchial chamber, and its great multi- 

 plication of gill-slits it suggests the successful attempt of an 

 animal to survive through the power of acquiring secondary 

 adaptations, the only one out of a large group that has come 

 down to us. The segmentally arranged muscles are essentially 

 vertebrate, and a series of rather complex nephridia in the 

 gill region may be homologous with a pronephros. Even those 

 mysterious organs of true vertebrates, the thymus, thyreoid, 

 and epithelial bodies, the history of which begins in the 

 cyclostomes with a series of segmental anlagen, are probably 

 seen here in an earlier stage, for it has been suggested that the 

 endostyle is the homolog of the thyreoid series, and the 

 numerous thymus anlagen have been rather hesitatingly identi- 

 fied with certain elements of the gill skeleton. 



