388 GILMAN A. DREW. 



Cerebral ganglion, cp. Cartilage pit. cs. Cliitinous support, ec. Ectoderm. 

 /. Foot. g. Gill, gs. Suspensory membrane of gill. h. Heart, int. Intes- 

 tine, ip. Inner plate of the gill. ilji. Inner labial palp. k. Kidney, /, 

 Liver, Im. Longitudinal muscle. Ip. Labial palp. m. Mantle, mg. Mid- 

 gut, mo. Mouth, oes. ffisophagus. olp. Outer labial palp. op. Outer 

 plate of the gill. ot. Otocyst. pa. Posterior adductor muscle, pap. Palp 

 a|)pendage, pas. Posterior adductor muscle-scar. pg. Pedal ganglion, pn. 

 Palp nerve, sg. Shell-gland, std. Stomodseum. sto. Stomach, t. Test, 

 tc. Cavities iu the mantle caused by teeth on the shell, v. An organ of un- 

 known function. vg. Visceral ganglion, y. Cut wall of gill plate, z. 

 Scattered cells of the disorganised liver. 



PLATE 20. 



Fig. 1.— Adult specimen with the brood-sac attached. The brood-sac is 

 torn open to show the eggs inside. X 10. 



Fig. 2. — Sixteen-celled stage. X 150. 



Fig. 3. — Section of an embryo in the sixteen-celled stage. X 275. 



Fig. 4. — Section of a later cleavage stage that corresponds to an epibolic 

 gastrula. The asterisk (*) marks the position where the gut is formed. X 

 275. 



Fig. 5. — An embryo that is slightly older than the one represented in sec- 

 tion by Fig. 4. X 150. 



Fig. 6.— Lateral view of an embryo in which the gut has been formed, 

 represented as a slightly transparent object. From the study of preserved 

 material I am inclined to think that the shell-gland does not bear cilia, but 

 this has not been determined on living material. The line marked 7 indicates 

 the plane in which the section. Fig. 7, was taken. X 150. 



Y\G. 7, — Transverse section of an embryo in the stage represented by Fig. 

 6. The line 7, on Fig, 6, indicates the plane of the section, x 275, 



Fig. 8.— Sagittal section of an embryo iu the stage represented by Fig. 6. 

 X 275. 



Fig. 9.— Sagittal section of an embryo slightly older than the embryo of 

 which Fig. 8 is a section. It represents the beginning of the formation of the 

 stomodeeum. X 275. 



Fig. 10. — Dorsal view of an embryo in which the test is growing over the 

 shell-gland. The lines numbered 11, 12, and 13 indicate the planes of sec- 

 tions represented in corresponding figures. X 150. 



Fig. 11.— Sagittal section of an embryo in the stage represented by Fig. 

 10. The line 11 on Fig. 10 indicates the plane of the section. X 275. 



Figs. 12 and 13. — Transverse sections of an embryo in the stage repre- 

 sented by Fig. 10. The lines 12 and 13 on Fig. 10 indicate the planes of the 

 sections. X 275. 



