888 



GILMAN A. DREW. 



Cerebral ganglion, cp. Cartilage pit. cs. Ciiitinous support, ec. Ectoderni. 

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

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

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

 gut, mo. Mouth, oes. (Esophagus, olp. Outer labial palp. op. Outer 

 plate of the gill. at. Otoeyst. pa. Posterior adductor muscle, pap. Palp 

 appendage, pas. Posterior adductor muscle-scar. pg. Pedal ganglion, pn. 

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

 tc. Cavities in 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. 



Fig. 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 in 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 

 stomodseum. X 275. 



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

 shell-gland. The lines numbered II, 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. 



