THE PHYLUM CHORDATA 



63 



and in their coloration, which is often quite striking, the prevailing 

 colors being brilliant yellows, red-orange tints, and various shades of 

 green. The most essential ana- 

 tomical chfferences between the 

 genera are concerned with the 

 degree of development of the 

 so-called notochord. 



A description of the more 

 significant anatomical details 

 of Balanoglossus will serve 

 as a characterization of the 

 group. 



The Notochord. — The struc- 

 ture which is identified as homol- 

 ogous with the true notochord 

 of typical chordates is identified 

 as such largely on account of 

 its relations to other structures. 

 It consists of a short thick- 

 walled diverticulum of the mid- 

 dorsal region of the anterior end 

 of the alimentary tract. The 

 diverticulum projects forward 

 as a rod into the cavity of the 

 proboscis and is stiffened by a 

 Y-shaped "proboscis skeleton," a 

 chitinous secretion of the sheath 

 of the diverticulum. The his- 

 tological structure of the noto- 

 chord is not unlike that of the 

 true notochord in that its cells 

 are vacuolated. The cUvertic- 

 ulum is therefore diagnosed as a 

 notochord by virtue of its deri- 

 vation from a median dorsal por- 

 tion of the alimentary tract and 



b'.c? 



Fig. 36. — Schizocardiwn brasiliense, 

 longitudinal, median section through the 

 anterior end. h, blood space; h. c', b. c', 

 h. c^, first, second and third body cavities; 

 cm, circular muscles of proboscis; e, epi- 

 dermis; hn, longitudinal muscles; m, 

 mouth; ?i, notochord; ns, central nervous 

 system; d, dorsal nerve; pc, pericardium; 

 ■ps, proboscis stalk; s, proboscis skeleton; 

 V, vermiform process or extension of noto- 

 chord. (From Harmer, after Spengel.) 



because it seems to serve some 

 skeletal function. Schizocardium (Fig. 36) has a much longer "noto- 

 chord" due to its extension forward into a long vermiform process. 



