352 KOREN AND DANIBLSSEX ON THE 



an embryo proceeding from a single ovum is constantly met with; 

 but this embryo never attains to a complete development. 



8. The number and size of the embryos vary in different cap- 

 sules ; the average number is from 20-40. The largest embryo 

 was 1^ millim. in diameter. 



9. The first organs w^hich are formed after the tegumentary 

 membrane are the foot with its vibratile cilia and the two 

 rounded lobes, w hich are both ciliated and provided with cirrhi ; 

 then the mantle, the shell and the auditory organs, the salivary 

 glands, the heart (on the twenty-third day), the eyes and the 

 tentacles. The digestive apparatus, the nervous system, the 

 branchiae, the siphon, and the retractor muscles of the foot, ap- 

 peared later still. Subsequently the heart divides into two 

 chambers, the shell presents one or two spiral turns, and it is 

 only after all these changes that the contractile bladder appears. 



After eight weeks the young had not yet left the capsules ; 

 and when one was extracted in this stage of development, it 

 began to creep like the adult animal, from which however it 

 was distinguished by the lobes, which had not wholly disap- 

 peared, and by the shell, which still had only one or two spiral 

 turns. 



10. Towards the ninth or tenth week, the young leaves the 

 capsule ; the lobes have disappeared ; the shell has become fra- 

 gile and opake. 



EXPLANATION OF THE PLATES. 



Buccinum undatum. Plate X. 



Fig. 1 . An egg from the oviduct of Buccinum, x 200. a, yelk ; h, germinal 

 vesicle ; c, germinal spot. 



Fig. 2. Egg from a capsule, X 200. a, chorion ; h, vitellary membrane ; 

 c, yelk. 



Figs. 3 & 4. Embryos, a, chorion ; 6, yelk with its membrane ; c, rudi- 

 ments of the two lobes. 



Fig. 5. Embryo seen from the side, a, membranous shell ; 6, mantle ; 

 c, yelk ; d, lobes ; e, foot. 



Fig. 6. Embryo. Dorsal view, f, heart. 



Fig. 7. Embryo. Ventral view. /, foot. 



Fig. 8. Embryo. Dorsal view. A, salivary glands. 



Fig. 9. Embryo. Ventral view. », tentacles; /.-, proboscis; /, oesophagus; 

 OT, stomach. 



