ILLUSTRATIONS. 89 



PLATE XV. 



(All the figures represent Isametra vinpara.) 



Fid. 1. Sagittal, longitudinal section of an embryo, in which the division of the archenteron into the ccelomic 

 vesicle and the entero-hydroecel has been completed. The mesenchynie cells have not yet 

 filled the blastocoel cavity completely in the lower part. The concavity on the left (ventral) 

 side represents the beginning formation of the vestibulary invagination. X165. 



FIG. 2. Part of the entoderm from the section represented in plate xiv, figure C. X535. 



Fro. 3. Part of the ectoderm from the section represented in plate xiv, figure 4. X535. 



FIG. 4. Transverse section of an embryo in which the parietal canal does not open outwards; it is not 

 distinctly apically elongated, being present in only five sections upon the whole, and thus 

 represents a comparatively younger stage of development than figure 5, as is also seen from the 

 fact that the vestibulary invagination is quite indistinct. In the mesoderm a fairly large 3 f olk 

 globule (y. g.) is seen. X165. 



FIG. 5. Transverse section of an embryo, showing the parietal canal opening outwards; the vestibulary 

 invagination is a slight but distinct concavity on the ventral side. X 165. 



FIGS. 6, 7. Two transverse sections of an embryo. The two sections figured are separated from one another 



by five sections. 



Figure 6 is a section from the posterior end, showing the two enteroccel vesicles surrounding the 

 entoderm separated from one another in the dorsal and ventral midline by a thin wall, the 

 mesentery. The entoderm with a mere indication of a lumen, otherwise filled with a yolk- 

 mass, containing nuclei. 



Figure 7. More anterior section, showing the hydrocoel cut in two places. The pore canal is indis- 

 tinct in this series; it is represented only by a compact mass of cells, no lumen being discernible. 

 The left enteroca-1 has disappeared in this section. Glandular cells are fairly numerous in 

 the ectoderm. X 165. 



FIG. 8. Transverse section, corresponding to that represented in figure 7; shows a very large globule of 

 yolk and some smaller ones in the mesoderm. In this embryo there is a rather large lumen 

 in the entoderm, in which a fine granulated mass is seen, probably dissolved yolk-mass. The 

 extraordinary outline of this figure, which has even been slightly corrected, is due to the fact 

 that the embryo has been irregularly compressed by the mutual pressure of the embryos 

 within the marsupium. X165. 



FIG. 9. Longitudinal, frontal section, showing the parietal canal opening outwards through the hydropore. 

 The hydrocoel is visible in this section only as a small mass of nuclei lying between the left 

 (oral) coelom and the pore canal. The vibratile bands are distinct. X 165. 



FIGS. 10, 11. Two longitudinal, frontal sections of an embryo. 



Figure 10. The more dorsal of the two, showing the beginning formation of the chambered organ 

 as prolongations from the right, aboral coelom. The nervous system is beginning to develop. 

 Figure 11. A more ventral section, separated from that figured in figure 10 by sixteen sections. 

 It shows the anterior prolongation of the parietal canal; no opening of the pore canal could 

 be discerned in this series. In the aboral ccelom the vertical mesentery has been formed. 

 The concavity of the anterior end of the embryo is the suctorial disk. The vibratile bands aie 

 distinct. X 165. 



PLATE XVI. 



(All the figures represent Isometra rivipara. All X1G5.) 

 FIG. 1. Transverse section of an embryo showing the formation of the parietal canal as an outgrowth from 



the hydroccel. The outline of the figure slightly corrected. 



FIGS. 2 to 5. From a frontal, longitudinal series of sections of a fully formed larva. Figure 2 is the more 

 ventral of the sections; nine sections lie between figures 2 and 3, four sections between figures 

 3 and 4, twelve sections between figures 4 and 5. 



Figure 2 is in the level of the outer opening of the vestibulary invagination. 



Figure 3 is at the bottom of the invagination, which is seen to be deeper in its anterior part The 

 different arrangement of the glandular cells and the nuclei around the invagination in figures 

 2 and 3 is to be noted. The notch in the anterior end in figure 2 is the suctorial disk. In 

 figure 3 the parietal canal and the apparently obliterating pore canal are seen. Parts of 

 the nervous system appear in the anterior end of this figure; in the posterior end some yolk- 

 globules are seen. 



Figure 4 shows the hydroca'l, from which the primary radial canals are about to develop. The dark 

 mass above is a group of glandular cells from the bottom of the vestibulary invagination 

 in transverse section. In the posterior end is seen a pair of large yolk-globules. The groups 

 of nuclei in the ectoderm of this and the following figure indicate the vibratile bands. 

 The cilia are indistinct in this series on account of the egg-membrane lying very close to 

 the epidermis. 



Figure 5 goes through the middle of the entoderm, showing the indistinctly limited lumen in the 

 middle of the mass of yolk-cells. The series of narrow lumina above the vertical mesentery 

 are due to the decalcified young stalk-joints. 



