66o 



COMPARATIVE ANATOMY 



in the other, the nervous system was dorsal to the alimentary canal, 

 leading to chordates. Balanoglossus, the representative genus of hemi- 

 chordates, belongs in the latter group and exhibits many traits which are 

 transitional between those of non-chordates and vertebrates. The found- 



APICAL PLATE 



COLLAR CAVITIES 



GILL SLITS^ 



CAVITY 

 DIVERTICULA) 



COLLAR>v^,l 



IRCULAR BAND 

 OF CILIA 



''TRUNK CAVITIES 



Fig. 533. — Balanoglossus embryos. .4. A horizontal section of a young embryo, 

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

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

 affinity of the.se 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.) 



ations of the balanoglossus theory of vertebrate ancestry are found by 

 Bateson in the presence of a rudimentary notochord, gill slits resembling 

 those of amphioxus, a dorsal nerve cord which is tubular in the anterior 



GILL-SUT 



•x.s. 



B. HARRIMANIA - STEREOGRAM OF COLLAR REGION. 



A. ESOPHAGEAL REGION - 



Fig. 534. — Harrimania, a Hemichordate. .4. The dorsal portion of a cross section 

 of Harrimania in the region of the esophagus. 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.) 



portion, a preoral lobe with epithelial cavities resembling those of the 

 pre-oral lobe of amphioxus, segmentally arranged gonads like those of 

 amphioxus, three pairs of enterocoelic cavities, and the resemblance of 

 the tornaria larva of balanoglossus to the early embryo of amphioxus. 

 All of these features show that the affinities of balanoglossus are with 



