No. 3.] GROWTH AND METAMORPHOSIS OF TORN ARIA. 417 



contains several gland cells which stain deeply, as shown in the 

 figure. At cb the section passes through the circular band. It 

 is seen to be composed of three rows of cells, each richly cili- 

 ated and with large nuclei. The protoplasm of the cell is often 

 striated at right angles to the surface, as though continuous with 

 the cilia. Immediately below the circular band is a pigment 

 spot. It is seen to be formed of a finely granular pigment area 

 surrounding a clear central spot, — a window, as it were, through 

 which the pigment may be seen. All of the pigment spots, both 

 on the longitudinal and circular bands, have this same structure. 

 In Fig. 42, PI. XXVII., another of these pigment spots is shown, 

 drawn to a larger scale. It is taken from the lower limb of the 

 longitudinal band, and has exactly the same structure as in the 

 preceding case. Whether the pigment is in the same cell that 

 has the clear portion in its outer end, or whether in other cells, 

 I could not determine from sections. Judging from the extent 

 of pigmentation, it seems probable, at any rate, that it must be 

 contained in more than a single cell. Returning to Fig. 36, we 

 see that the ectoderm over the posterior (oral) pole of the 

 embryo is quite thick, though not so much so as in the extra- 

 oral area. In the centre of this plate lies the anus, where the 

 ectoderm runs in, and becomes immediately continuous with 

 the endoderm of the intestines. (The anus is not seen in the 

 figure.) The central part of the section is largely filled by 

 the digestive tract. The diaphragm-like partition separating the 

 stomach (S) from the intestine (/) is conspicuous. It is seen 

 to be formed by a fold in the walls of the mid-gut, and there- 

 fore two-layered. There is formed in the centre of the dia- 

 phragm a small opening, around which the large cells are richly 

 ciliated with exceedingly long cilia. During life this bunch of 

 cilia is seen in constant action, often with a rotary motion, 

 churning up the contents of the stomach. The cells lining the 

 stomach division are tall and columnar, and are not ciliated 

 (except at one place, to be spoken of later). The cells of the 

 intestine are not so tall, and often show, as in the figure, a 

 plasmodium-like outline. The second and third body cavities 

 are seen at the sides of the digestive tract. Each is a sac with 

 walls composed of a single layer of cells surrounding a small 

 central cavity. The somatic wall is formed from higher cells 

 than the splanchnic. 



