2IO CHORD AT A 



Vertebrates, pointing out the distinguishing features which mark 

 an advance in the Vertebrate type. 



Invertebrates versus Vertebrates 



Dorsal heart. Ventral heart. 



Solid nervous system. Tubular dorsal nervous system. 



No endoskeletal structures. Either a notochord or a vertebral column. 



There is no Class of the Invertebrate Metazoa whose characters 

 have not been repeatedly suggested to reveal affinities with the 

 Vertebrates. 



Before discussing the different theories of Origin of the Verte- 

 brates, it will be best to take up the anatomy of examples of the 

 Hemichorda, Urochorda and Cephalochorda, since these are con- 

 sidered by some to be connecting types. 



Subphylum Hemichorda. (Enteropneusta.) Characteristics. 

 (Balanoglossus.) — i. Hollow, tubular notochord, opening into a 



straight alimentary canal. 



2. Gill slits. 



3. Dorsal blood vessel and anterior 

 (dorsal) heart. 



4. Dorsal and ventral nerve strands, 

 with many nerve fibers and scattered giant 

 nerve cells. 



The genus Balanoglossida includes 



Balanoglossus {Dolichoglossus) kowalevskii. 



This form is divided into three regions, 



the proboscis^ the collar, and the trunk. 



The proboscis, when distended with water, 



serves as an organ for burrowing in the 



mud. Gill slits open into the anterior end 



of the straight alimentary canal. The 



paired hepatic cecae are located near the 



posterior end of the digestive tube. The 



dorsal and ventral blood vessels receive 



blood from the anterior heart. Excretions 



are extracted by the kidney, which sends 



its wastes out through the proboscis-pore, 

 Doscis; 2, collar; ?, giu-siics. . , , n 1 n • j t-i 



(After A. Agassiz. Courtesy With Other expelled fluids. The sexes ^ are 



of Henry Holt & Co.) separate and the ger7n cells, formed in a 



Fig. 98. Balanoglossus. 

 Linville and Kelly. /, pro- 

 boscis; 2, collar; j, gill-slits. 



