368 



LARVAE OF THE TRIPLOBLASTICA. 



ing of the mouth, and usually extending posteriorly to the 

 opening of the anus, when such is present. 



The dorsal dome is continued in front of the mouth to form 

 a large prceoral lobe. 



There is usually present at first an uniform covering of cilia ; 

 but in the later larval stages there are almost always formed 

 definite bands or rings of long cilia, by which locomotion is 

 effected. These bands are often produced into arm-like pro- 

 cesses. 



The alimentary canal has, typically, the form of a bent tube 

 with a ventral concavity, constituted (when an anus is present) 



FIG. 218. DIAGRAM OF AN EMBRYO OF PLEUROBRANCHIDIUM. 



(From Lankester. ) 



/. foot; ol. otocyst; m. mouth; v. velum; ng. nerve ganglion; ry. residual yolk 

 spheres; shs. shell-gland; i. intestine. 



of three sections, viz. an oesophagus, a stomach, and a rectum. 

 The oesophagus and sometimes the rectum are epiblastic in 

 origin, while the stomach always and the rectum usually are 

 derived from the hypoblast 1 . 



To the above characters may be added a glass-like trans- 

 parency ; and the presence of a widish space possibly filled with 

 gelatinous tissue, and often traversed by contractile cells, 

 between the alimentary tract and the body wall. 



1 There is some uncertainty as to the development of the oesophagus in the 

 Echinodermata, but recent researches appear to indicate that it is developed from the 

 hypoblast. 



