90 



SOLENOCONCHA. 



2. The Development of the Form of the Larva. 



As early as the gastrula-stage, the embryo becomes free and is 

 capable of active locomotion, some of the ectoderm-cells being already 

 covered with cilia (Fig. 34 'C). Besides those ciliated cells which lie 

 at the cephalic pole and later form the ciliated tuft, the young larva 

 has three rows of such cells lying one behind the other at the middle 

 of the body of which they form a large part (Figs. 34 C and 35 ^4). 

 Since these ciliated cells represent the pre-oral ciliated ring, the post- 

 oral part of the larva is very little developed. At this early stage, 

 the larva consists of comparatively few cells, which are still very large, 

 as is evident from a glance at Fig. 35 A. In later stages, as the larva 

 grows in size and as its cells increase in number, the rows of ciliated 



jnoe. 



FIG. 35. A-C, three larvae of Dentalium aged respectively 12, 24 and 37 hours (after 

 KOWALEVSKY). bl, blastopore ; m, mantle-fold ; moe, permanent posterior aperture 

 of the mantle ; p, posterior part of the body ; w, ciliated ring ; ws, apical ciliated 

 tuft. 



cells are less conspicuous as compared with the rest of the body (Fig. 

 36 A), and finally appear as a single though somewhat broad ciliated 

 ring (Fig. 35 A-C). Meantime, the ciliated tuft at the cephalic pole 

 has become more conspicuous, and a large part of the anterior section 

 of the body has also become covered with delicate cilia (Fig. 35 B). 

 In the youngest larvae, niz., at the gastrula-stage, the blastopore 

 was terminal, i.e., opposite to the cephalic pole (Fig. 34 C), but it 

 soon changes its position, shifting forward towards the ciliated ring- 

 along the future ventral surface (Fig. 35 A). The larva thus 

 assumes a somewhat irregular shape, the flattened ventral surface 

 being somewhat backwardly inclined. At the same time, the pre- 

 oral part of the larva, that lying in front of the ciliated ring, has 



