4 AMPHINEURA. 



are specially large (C, m). These cells which, as well as others 

 situated near them, at first lie in continuity with the entoderm, 

 represent the rudiment of the mesoderm. They soon shift out of the 

 series of entoderm-cells into the cleavage-cavity (D, m). The meso- 

 derm-rudiment which thus arises seems at first to have a regular 

 bilateral arrangement in keeping with its origin, i.e., two groups of 

 large cells lying near the blastopore can be seen, but this regularity 

 is soon lost, the cells, which subdivide further, becoming scattered. 

 In this respect, and perhaps also in the manner of its origin, the 

 mesoderm of Chiton may be compared with that of other Molluscs 

 (Lamellibranchia, p. 29, Gastropoda, p. 117). 



2. The Development of the Larval Form. 



Even before the development of the germ-layers has progressed 

 thus far, alterations take place in the external shape of the embryo. 

 Two adjacent rows of cells in the ectoderm of the gastrula become 

 distinguished from the rest as bearing cilia (Fig. 1 (7, iv), and these 

 divide the larva into an anterior and a posterior section. Similarly, 

 a group of cells lying at the pole furthest away from the blastopore 

 becomes covered with cilia. These two groups of ciliated cells are 

 the rudiments respectively of the ciliated ring [velum] and of the 

 frontal or apical ciliated tuft (Figs 2 and 3, w and ws). Very similar 

 embryonic stages are met with in the ontogeny of other Mollusca, e.g., 

 Patella (Fig. 50, p. 124). The pre-oral ciliated ring in the Lamelli- 

 branch larvae is also formed of two rows of cells. Indeed, the 

 ciliated ring seems usually to be biserial ; though, in Patella, there 

 are three rows of cells (Figs. 52 and 53, pp. 126, 127). 



As the body extends in the direction of its principal axis, the blas- 

 topore, which has hitherto lain at the posterior pole, assumes another 

 position and form. It shifts to that side of the larva which later 

 becomes the ventral surface, and, owing to the growth of the dorsal 

 surface, gradually approaches the ciliated ring (Fig. 1 JB-D). The 

 blastopore, as it shifts its position, loses its circular form, and, as far 

 as we can make out from the figures, assumes the form of a trans- 

 verse slit (Fig. 3 B). Meantime, the continuous growth of the dorsal 

 surface causes the aperture to shift more and more towards the 

 ciliated ring, and it is finally found immediately behind it (Fig. 2 A). 

 This slit-like aperture, however, no longer corresponds fully to the 

 blastopore, since the ectoderm surrounding the latter has sunk below 

 the surface, and the actual primitive mouth thus comes to lie at the 

 inner end of a laterally compressed ectodermal tube which for some 



