CHORDATE ANATOMY 



APICAL PLATE 



GILL SLITS^ 



P/IC 



(ANT ENDO. DIVERTICULA) 



COLLAR CAVITIES 



NOTOCHORD 



PROBOSCIS 



IRCULAR BAND 

 OF CILIA 



^TRUNK CAVITIES 



Fig. 5. — Balanoglossus embryos. A. A horizontal section of a young embryo, 

 showing the origin of mesodermal pouches. McBride and others have noted the 

 similarity of this section to that of a young Amphioxus embryo as evidence of the close 

 affinity of these two forms. B. A young Balanoglossus larva with five pairs of gill-slits, 

 viewed from the left side. The gill-slits of Balanoglossus bear a striking resemblance 

 to those of Amphioxus. On the other hand the young larva of Balanoglossus is strik- 

 ingly like the larva of echinoderms. (Redrawn after Bateson.) 



.GILL-SLIT 



A ESOPHAGEAL REGION - X.& B. HARRIMANIA - STEREOGRAM OF COLLAR REGION. 



Fig. 6. — Harrimania, a hemichordate. A. The dorsal portion of a cross section 

 of Harrimania in the region of the esophag\is. The resemblance of this cross section 

 to one of Amphioxus is striking and serves to demonstrate the close genetic affinities 

 of these two chordates. In Harrimania the notochord is present not only in the preoral 

 lobe as in other hemichordates but also in the collar and anterior pharyngeal regions. 

 B. A stereogram of Harrimania in the collar and anterior pharyngeal region, showing the 

 presence of the notochord in these regions. Such evidence tends to remove the doubt 

 that a true notochord exists in hemichordates. (Redrawn after Ritter.) 



PROBOSCIS PORE 



ARMSr 



NERVE STRANO 



.GONAD 



'PHARYNX 

 IPHARYGEAL POUCH 



- STALK 



>^-Ji^ I 1 ; *^ " 



notochord' MOUTH' 



CEPHALOOISCUS. 



Fig. 7. — A diagram of Cephalodiscus viewed from the left side as if in median 

 optical section. The presence of a notochord in the pre-oral lobe is one of the reasons 

 for placing this animal among hemichordates. While not regarded as a form " ancestral 

 to vertebrates, Cephalodiscus interests morphologists as a primitive chordate. (Redrawn 

 after W. Patten.) 



