658 



COMPARATIVE ANATOMY 



theory is that it derives vertebrate from invertebrate without reversing 

 dorsal and ventral sides. Hubrecht assumes that the paired nerve 

 cords of the Nemertean unite in the mid-dorsal line to form the nerve cord 



of vertebrates. The paired cere- 

 bral lobes of the nemertean become 

 the brain of the vertebrate. 

 Hubrecht compares the intestinal 

 diverticula of the flatworm with 

 the coelomic outpocketings of the 

 amphioxus embryo. The more 

 anterior diverticula are possibly the 



Fig. 530. — Diagrams illustrating Hu- r 1 -n 1 r 



brecht's theory of the origin of the noto- homologues of the gill pouCheS ot 



chord from the proboscis sheath of rhordates 



nemerteans. A, diagram of a nemertean, ,■ ■, i • r j-rc 



B. of a chordate. The proboscis of the One of the chief difficulties in 



nemertean according to Hubrecht becomes making Comparison between 



the vertebrate hypophysis. ° 



vertebrates and invertebrates has 

 been the absence in invertebrates of any structure similar to the 

 notochord. By comparing the proboscis sheath of nemerteans with 

 the notochord, Hubrecht thinks that he has removed this obstacle 



ECTODERM 



CIRCULAR 

 MUSCLES 



DORS NERVE-CORD 

 LONG. MUSCLES 



PROBOSCIS SHEATH 

 V- PROBOSCIS 



r GONAD 



LATERAL 

 NERVE-CORD 



LATERAL 

 BLOOD-VESSEL 



PARENCHYMA 



NEMERTEAN-CROSS SECTION 

 Fig. 531. — A cross section of a Nemertean. While the nemertean lacks a coelom, 

 the gonadic sacs have been compared with the coelomic sacs of coelomate animals. 

 (Redrawn after Lang.) 



and has thus demonstrated that nemerteans are the connecting link. 

 The nemertean proboscis, which is of ectodermal origin, is homo- 

 logized with the vertebrate hypophysis. Hubrecht points out that the 



