ECHINOCYAMUS PU8ILLUS. 43 



front, fig. 82 — 83 n. Thus, the intestine lies pressed between the sto- 

 mach and the ventral wall of the larva. 



In the full-sized Pluteus of this first stage even the ectoderm has 

 changed remarkably. In the Gastrula it consisted of a rather uniform 

 cylinder-epithelium except at the animal pole of the larva, where it at- 

 tained a considerable height. A few days afterwards, when the larva is 

 about three or four days old, the ectoderm has become very thin, except 

 at the ciliated band, and is made up of a simple layer of large poly- 

 gonal flattened cells with distinct nuclei. A section through the larval 

 body-wall shows that the cells resemble much flattened scales with a 

 slight dilation where the nucleus has its place, PL IV, fig. 59 — 60. 

 Owing to the size of these cells and the fact that they each carry only a 

 single cilium, the cilia become rather sparse on the surface of the larva. 

 Scattered among the above polygonal cells, ethers are to be found 

 which present a round discoidal form, PL IV, fig. 58, and are marked 

 by a number of dots, which treated with nitrate of silver become very 

 well defined and when seen in profile appear as small knobs. By the 

 use of the same staining reagent we find that the surface also of the 

 polygonal cells appears finely punctuated. Is seems probable that this 

 punctuation is due to minute plasmatic processes shooting out from the 

 cells and that the larger dots or rather knobs in the rounded cells are 

 nothing but bundles of such processes, which surround the cilium. In- 

 deed, as I have already said, I have often observed that the cells are 

 provided with not only a large cilium but also with minute hair-like 

 processes which give the surface of the cells a finely fringed appearance. 



The cells which constitute the ciliated band are remarkable. They 

 present themselves in the shape of a bottle bearing outwardly, besides 

 a cilium, a considerable border, PL /F, fig. 61 — ff.5, and when examined 

 with the highest power, they curiously remind one of collar-cells in the 

 Porifera. Each cell is produced at its free extremity into a long cilium, 

 round the base of which the clear ectoplasm of the cell protrudes like a 

 collar, which sometimes appears homogeneous, but often on the other hand 

 is finely striated, as if it were composed of fine ectoplasmic processes. 

 All these minute projecting parts of the ectoplasm of the cells whether 

 they are scattered as in the polygonal flattened cells, or collected in 

 bundles as in the discoidal cells, or united so as to form a collar, 

 ofi"er great interest and have as far as I know escaped the attention 

 of other investigators. They possibly explain the way in which the 

 collar is formed in the true collar-cells in the Porifera. At first, there 



