1897.] NATURAL SCIENCES OF PHILADELPHIA. 159 



The formation of the shell was observed as follows : On July 29th 

 at 8.30 P. M. the larva swam slowly about a limited area for about 

 fifteen minutes. It then remained quiet and attached itself to the 

 glass in which it had been placed, after a few minutes it began to 

 secrete a translucent mass about it. At 9.30 P. M. the shell thus 

 formed presented a translucent horn-shaped appearance and formed 

 a permanent tube covering about one-half of the fully extended 

 body. The upper half of the body was constantly protruded from 

 and withdrawn into this half formed shell. 



In the ninth stage the shell assumes a spiral form, but is about 

 half the diameter of the adult shell. The annelid still possesses 

 apical ocelli ; the collar disappears ; the tentacles are filiform and 

 branched. By differentiation of the cephalic region, and by growth 

 of the larva in length of the posterior part of the body, and by seg- 

 mentation into numerous metameres, the originally unsegmented 

 larva is transformed into the adult annelid. 



The time occupied by the development of S. borealis from the 

 first segmentation stage to that of the free swimming ciliated larva 

 was found by a series of observations to occupy the space of three 

 days. Two days later the annelids attached themselves and com- 

 menced the formation of shells ; the length of time required from 

 this stage on to completion of the adult shell has not been ob- 

 served. 



BIBLIOGRAPHY. 



A. Agassiz. Spirorbis spirillum, Annals of Lyceum of Natural 

 History, Vol. VIII, pp. 318-323. 



J. W. Fewkes. Spirorbis borealis, American Naturalist, March, 

 1885, p. 249. 



Pagenstecker. Spirorbis spirillum, Zeitschrift fiir wiss. Zoologie, 

 XII. 



EXPLANATION OF PLATES I AND II. 



Figures 1 to 10. Segmentation stages. 



Figure 11. Blastula stage. 



Figure 12. Gastrula stage. 



Figure 13. Sixth larval stage. Ventral view. (Still in the 



egg capsule). 

 Figure 14. Seventh larval stage. Ventral view. (Ready to 



escape from egg capsule). 

 Figure 15. Early shell secreting stage. Ventral view. 



