346 A. T. MASTERMAN. 



On the ventral side the ectoderm consists of long glandular 

 cells closely crowded together, and sometimes thrown into 

 folds. They are covered with minute cilia, and are continuous 

 ventrally with the epithelium lining the mouth-cavity. 



Ventro-laterally are given off, from each plume, the rows of 

 pinnae on each side. A cross-section of a pinna is shown in 

 fig. 27. The dorsal and ventral surfaces of the pinna are 

 covered by similar cells respectively to the same two surfaces 

 of the plume, and under the dorsal cells is a crescentic (in 

 transverse section) blood-vessel whose lumen is continuous 

 with that of the plume. The rest of the internal cavity is 

 coelomic, and is also continuous with that of the plume. A 

 few intersecting strands of mesoderm are to be seen. There 

 does not appear to be any definite nerve in the pinna. 



The way in which the six plumes are arranged is peculiar, 

 and although one or two may be, and usually, in sections, 

 are slightly displaced, it is easy to demonstrate the principle 

 upon which they are disposed. PI. 26, figs. 32 — 36, are 

 meant to illustrate this, and they are selected from a series 

 of sections cut perpendicular to the long axis of the buccal 

 shield. 



The areas occupied by glandular ciliated epithelium, and be- 

 longing morphologically to the ventral surface, are indicated 

 by the thicker lines, whilst the thinner lines indicate the 

 morphological dorsal surface with its characteristic non-ciliated 

 epithelium. In fig. 32 the right side shows the base of the 

 united plumes and the post-oral lamella behind them, whilst 

 the left side shows indications of a separation of the two most 

 posterior plumes (5, 6). In fig. 33 these two are seen to be 

 free from the main axis, and very little further up (fig. 34) the 

 next (4) branches off". Higher up still these three arrange 

 themselves in linear series, so that their morphological dorsal 

 surface is opposite to that of the main axis, which has not yet 

 divided into three. Fig. 35 shows the separation of (3), and 

 further up again the last two diverge. Fig. 36 indicates the 

 arrangement some way further towards the tips, and the six 

 plumes are here seen to be arranged in more or less of a circle, 



