ON THE DIPLOCHORDA. 299 



oral hood is an area of epiblastic cells which bear long and 

 prominent cilia. The cells themselves are rather longer than 

 broad (PI. 21, fig- 44), and fine fibrils are readily distinguished 

 passing into the mass of fibres which forms a nerve-ring 

 immediately under the epiblast. These cells come to a sudden 

 termination at the posterior angles of the pre-oral lobe, where 

 they are continued on to the ordinary cells of the body. 



Tentacles. — The epithelium of the tentacles is of two 

 types. PI. 19, fig. 17, shows a succession of tentacles cut in 

 transverse section, so that various stages are indicated. Starting 

 from the right-hand side the tentacles are seen to arise as pro- 

 tuberances of the thickened ciliated cells of the collar area, 

 with a diverticulum of the collar coelom running down the 

 centre. The nervous area at the base of the cells is prominent, 

 especially at the outer angle of the thickened cells. As the 

 tentacle becomes completely separated from the surface of the 

 collar its inner wall is completed by a very thin non-ciliated 

 area of cells. The transverse section of a tentacle (PI. 21, fig. 

 43) is therefore somewhat ovoid, two-thirds of its epithelium 

 having thickened ciliated cells, and the posterior one-third 

 consisting of a thin cellular lamella. At the outer angle is a 

 nervous area, which is probably a tentacular nerve seen in cross- 

 section; and immediately internal to this is a hsemocoele space 

 or vascular trunk, which, however, is only present at the base 

 of the tentacle (cf. fig. 43). The rest of the cavity is coelomic, 

 and is lined by a mesoblastic wall, between which and the 

 epiblast is a mesoblastic skeletal lamella, referred to later. 



The Perianal Band. — The perianal band is composed of a 

 dense aggregation of very long columnar cells (PI. 18, figs. 1 

 and2,jo. a.) in direct continuity with the epiblastic cells of the 

 trunk. In cross-section the band appears lenticular in outline 

 (PI. 20, figs. 18 to 23, and PI. 21, tig. 45),- and the cilia appear 

 to be grouped more or less into masses. They are many times 

 longer than any other cilia on the body, and are slightly 

 curved towards the posterior end. Immediately in front of the 

 band is a nerve-ring in a shallow depression, and fibres from 

 this can be seen to enter the bases of the ciliated cells. 



