PHYLUM CIIORDATA 



387 



but only two genera, Cephalodiscus (Fig. 336) 

 and Rhabdo pleura (Fig. 335), belong to the order 

 CEPHALODISCIDA. 



The external features of one of the BALANO- 

 GLOSSIDA are shown in Figure 332. Three 

 regions may be distinguished: a proboscis (/), 

 a collar (2), and a trunk (j). Paired lateral 

 gill-slits (5) are present in 

 the anterior part of the 

 trunk. The mouth opens 

 on the anterior surface of 

 the collar region (4), and 

 the anus is situated at the 

 posterior end of the trunk. 

 The proboscis and collar pos- 

 sess cavities which become 

 filled with water through 

 ciliated pores (Fig. 333, 8). 

 When in a swollen condition, the proboscis and collar are forced 

 into the sand or mud, and constitute effective burrowing 

 instruments. 



Figure 333 shows diagrammatically the principal internal 

 structures of Glossobalanus. The notochord (//) is a supporting 



FIG. 332. Dolichoglossus kowalevskii. 

 i, proboscis; 2, collar; j, trunk; 4, mouth; 

 5, gill-slits. (From Shipley and Mac- 

 Bride, after Spengel.) 



FIG. 333. Longitudinal section through the middle line of Glossobalanus. 

 T, proboscis; 2, collar; 3, trunk; 4, proboscis cavity; 5, glomerulus; 6, peri- 

 cardium; 7, heart; 8, proboscis pore; Q, collar cavity; 10, mouth; //, noto- 

 chord; 12, dorsal blood-vessel; 13, oesophagus; 14, branchial rc-gion of ali- 

 mentary canal; 15, ventral blood-vessel; 16, gill-slits; 17, central nervous 

 system; 1-8, dorsal roots of nervous system; IQ, ventral pocket of proboscis 

 cavity. (From Shipley and MacBride.) 



