262 



SYSTEMATIC ZOOLOGY 



together particles of sand, thus forming a sort of tube for the 

 animal. 



The digestive tract is a relatively simple, straight tube, 



L 





- ; — - 



FlG. 271. Cephalodiscus. A single individual, enlarged. (After Mcintosh, from Parker 



and Haswell's Text-book.) 



terminating in the anus at the posterior tip of the bodv. Of 

 the other internal organs we need merely note that there is a 

 structure believed to be homologous with the notochord, which 



