292 A. T. MASTEEMAN. 



forms, and in preserved specimens is greatly folded on itself. 

 On either side of the middle line, near the nerve-ganglion, a 

 pore leads by a short canal into the coelomic cavity of the pre- 

 oral lobe. These two pores and canals are described in further 

 detail below, and seem to be comparable with the " proboscis- 

 pores^^ of Cephalodiscus. 



The ventral surface of the pre-oral lobe has an epithelium of a 

 very different character from that already describedfor the dorsal. 

 The body-wall is extremely thin, and consists of attenuated 

 cells (PL 19, fig. 6, and PI. 20, figs. 18, 19, 20, 28, and 29), 

 except in the region immediately an terior to the mouth. 

 Here they are rathe rmore cubical and bear cilia (fig. 6, v. 

 c. b.). I have not been enabled to detect any cilia upon the 

 rest of the ventral surface of the pre-oral lobe. 



Almost the whole surface of the collar segment is covered 

 with cilia, and the cells are fairly thick and cubical. Running 

 down the mid-ventral line from the mouth region is found a 

 ridge of cells with strong cilia, and on either side of this ridge, 

 for some way backwards, the cells of the ventral surface of the 

 collar are not ciliated, and are somewhat attenuated. Dorsally 

 the thick epithelium of the pre-oral lobe is continued on to 

 that of the collar on either side of the nerve-ganglion, and 

 (PI. 19, fig. 7) the dorsal surface of the tentacles is also 

 covered with thick epithelial cells bearing long cilia. 



In the larger larvae the epiblast of the trunk becomes so 

 attenuated as to defy complete analysis with the highest ob- 

 jective at my disposal (PI. 21, fig. 39). The only exception to 

 this is the epiblast of the perianal band, which is formed by the 

 longest cells of the whole body, aggregated in enormous num- 

 bers, and bearing long flagelliform cilia. Seen in cross-section 

 the ciliated ring or band appears lenticular, and just anteriorly 

 to it the cells are slightly thickened, and form a nervous ring 

 (PI. 20, figs. 22 and 23). 



This attenuation of the epiblastic cells of the trunk segment 

 seems to be attended by and connected with an elongation of 

 this region. Gegenbaur (9) has noticed that in the earlier 

 stages ('35 mm. long) the trunk region was short, and that by 



