236 ARTHUR WILLEY. 



times it is locally obsolete, ant] sometimps it is tiiiusually capa- 

 cious. In all cases it is patently vestigial. In more than 

 one of my species (e.g. Pt. cavnosa, n. sp.) the slomochord 

 undergoes fragmentation in the nuchal region, owing to the 

 invasion of skeletal substance. In Balanoglossus cana- 

 densis Spensel found that the entire nuchal region of the 

 stomochord was lacking. 



The structure of which the stomochord, in its capacity as a 

 portion of the gut, is a vestige, must originally have been 

 post-oral,^ and I liave convinced myself that the structures 

 recently described by Masterman ^ in the Actinotrocha of 

 the Bay of St. Andrews are capable of being explained on this 

 basis. 



In its middle region the stomochord is greatly dilated, both 

 dorso-ventrally and laterally. Spengel only emphasises the 

 ventral csecuru of the stomochord in this region ; but often 

 there is a pair of lateral pouches which are j)articularly well- 

 marked. In some cases the lateral pyuches are less pronounced 

 than the ventral caecum, and sometimes the exact reverse is 

 the truth. Sometimes the lateral pouches do and sometimes 

 they do not unite with one another across the middle line by 

 the intermediation of the ventral caecum. 



The interest of the situation has been increased rather than 

 diminished by the recently published observations of Roule on 

 the Actinotrocha of Phoronis sabatieri. Instead of the 

 paired lateral diverticula or pleurochords described by Master- 

 man, there is in Roule's larva a single median anterior ventral 

 diverticulum, whose cells likewise undergo vacuolar degenera- 

 tion, which gives it a sen)i-rigid consistency. According to 

 Roule this ventral diverticulum arises at the anterior end of 

 the intestine (in the same region as Masterman's pleurochords), 

 and projects forwards ventrally below the oesophagus. 



' I think it is universally admitted tiiat the stomochord is a socoudary 

 projection into the proboscis cavity pushing the ccelomic epithelium or 

 splanchuotheca ol' iSpeugel before it. 



- A. T. Masterman, "On the Diplochorda," 'Quart. Jouru. Micr. Sci., 

 vol. 40, 1897, p. 281. 



