c.m. 



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 differences 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 ftie cavity of the 

 proboscis and is stiffened by a 

 Y-shaped "proboscis skeleton," a 

 chitinous secretion of the sfieath 

 of the drverticulum. The his- 

 tological structure of the noto- 



a. 



36. Schizocardiwn brasiliense, 



true notochord in that its cells 

 are vacuolated. The divertic- 



f ,1 longitudinal, median section through the 

 chord is not unlike that of the Jj^ en ^ 6> blood space; &.c? &.c, 



b. c 3 , first, second and third body cavities; 

 cm, circular muscles of proboscis; e, epi- 



Tr v dermis; lm. longitudinal muscles; m, 



ulum is therefore diagnosed as a mouth ! n> n ;> toch( f rd; nS) central ne ; vou ^ 



notochord by virtue of its deri- system; d, dorsal nerve; pc, pericardium; 

 vation from a median dorsal por- P, proboscis stalk; s, proboscis skeleton; 

 , . f , -, v , , i 'v, vermiform process or extension of noto- 



tion of the alimentary tract and cnord (Fron ; 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> ] ong vermiform process. 





