ON THE DIPLOCHORDA. 301 



possibly to the liypophysis of the Vert eb rata. Its relation 

 to the blood -sy tern, &c., will be described under that head. 



Organs of the Hypoblast. 



The alimentary canal is ciliated throughout, consisting of a 

 tube of varying character, from the ventral mouth to the 

 posterior anus. The mouth opens immediately under the pre- 

 oral lobe, between it and the collar region. A mid-ventral 

 ciliated area leads into it from the pre-oral lobe in front, and 

 a broad ciliated area, depressed into two oral grooves (PI. 20, 

 fig. 29, 0. g.), approaches it from the ventral surface of the 

 collar area. 



From the dorso-lateral corners of the mouth lead outwards 

 the atrial grooves. The general arrangement, as seen in ventral 

 view, is indicated in woodcut 5. At the edge of the mouth 



5. View of mouth-parts of Actinotroclia, from ventral surface. In the oral 

 grooves the arrows point towards the mouth, in the atrial grooves they 

 point away, and indicate the course of currents. 



the cells on all sides pass by gradation to the long columnar 

 ciliated cells of the oesophagus. The nuclei of these are evenly 

 arranged at their centre (PI. 19, fig. 8), and the inner half of 

 each cell is clearer, and not so readily stained as the outer. 



In fig. 7 may be noticed the base of the subneural gland, 

 leading off from the " cesophagus ; '^ and in figs. 9 and 10 there 

 is shown the typical cross-section of the oesophagus below the 

 mouth. On their outer face the cells are covered by a thin 



