THE CEPHALOCORDA 177 



buccal folds is modified and drawn out into a number of 

 richly ciliated finger-shaped processes. The combined action 

 of the cilia causes a flow of water towards the mouth and gives 

 the appearance of a rotatory movement, hence the structure 

 has been called the wheel-organ. Close to it, lying against the 

 right side of the notochord, is a ciliated depression known as 

 Hatschek's organ. 



At the back of the vestibule is a nearly vertical partition 

 called the velum, perforated in the centre by the circular 

 aperture of the mouth. The mouth can be closed by a ring of 

 muscle fibres which surround it, and is further protected by 

 twelve oral tentacles which may either project freely backwards 

 into the pharynx, or may be folded across the mouth opening in 

 the manner shown in fig. 42, B. The oral tentacles are 

 provided with patches of sensory epithelium like those on the 

 buccal cirrhi, and when folded across the mouth they form a 

 very efficient strainer. Care should be taken not to confuse 

 the oral tentacles with the buccal cirrhi. 



The alimentary canal, into which the mouth leads, is a 

 straight tube running to the anus. A little distance in front 

 of the atriopore it is produced ventrally into a simple finger- 

 shaped diverticulum which is directed forwards and applied 

 closely to the right side of the pharynx. This diverticulum 

 ends blindly in front, and is lined with long columnar epithelial 

 cells : it appears to function as a liver ; at all events, the blood- 

 vessels have relations to it similar to those existing in the livers 

 of higher forms, and it is known accordingly as the hepatic 

 caecum. The caecum appears to be in the atrial chamber, but 

 it is really excluded from it by a complete investment of atrial 

 epithelium and mesoblast, representing the body-wall pushed 

 out in front of it as it grew forwards from the ventral side of 

 the gut. 



The section of the alimentary canal lying in front of the 

 liver caecum, extending from about the eighth to the twenty- 

 sixth myomere, is known as the pharynx. It is compressed 

 from side to side, but of considerable depth from above 

 downwards, and its side walls are perforated by a great 

 number of elongated gill-slits, which place its cavity in 

 communication with the atrial chamber. There may be as 

 many as 180 of these gill-slits on each side in a fully developed 

 specimen, but in young forms they are far less numerous. 



II. M 



