9 



May 7, at 6 p.m., and May 8, at 8 a.m., escaped May 25, at 10 

 a.m., or in seventeen and one half to eighteen days. In other 

 instances observed, twenty-one days passed before the outer case 

 was ruptured and abandoned by the embryos. Eighteen days 

 appears to be the more normal period for the embryo to remain 

 within the case; a longer period is due to adverse conditions. 



Embryos twelve and thirteen days old, forcibly removed from 

 the egg-case and allowed to escape into the surrounding water, 

 appear to suffer no ill effects from the premature change. One 

 specimen so treated lived for five days and developed normally. 



A very large proportion of the eggs laid develop to the stage 

 where the embryo is ready to escape from the egg-case, even 

 under the most adverse conditions. The majority of the egg- 

 cases observed were laid in very impure water containing great 

 quantities of infusoria and bacteria. Although this water was 

 rarely changed, over 99% of the eggs laid developed into embryos 

 able to escape from the egg-case. The egg-cases deposited on 

 microscope slides were subjected to extremely severe handling. 

 In order to remove diatoms and debris from the outside of the 

 outer case, that the developing embryo might be observed, it was 

 necessary to brush them at frequent intervals with a camel's-hair 

 brush. Occasionally a slide would be removed from the water 

 and the egg-case exposed to the air for fifteen minutes or more. 

 In spite of this treatment nearly all of the eggs on the microscope 

 slides lived to escape from the egg-case. 



When ready to leave the egg-case the animal is well developed, 

 and able to crawl by means of the foot in any direction. The 

 shell (Plate 2, fig. 10) is perfectly formed and measures 1 mm. in 

 length by 0.7 mm! in width. A small area at the apex of the shell 

 is smooth. The rest of the shell is minutely, densely, radiately 

 sculptured with narrow sharp-incised lines, regularly separating 

 broad raised ridges. The ridges, highly magnified, appear to be 

 formed of low irregular broad scales. No well-defined growth- 

 lines are to be seen at this age, nor any traces of concentric sculp- 

 ture. The mature shell shows no trace of radiating lines. The 

 jaw (Plate 2, fig. 11) composed of many overlapping plates, is 

 identical with that of a mature specimen excepting in size. The 

 radula (Plate 2, figs. 12, 13) is also well developed. It is 0.3 mm. 

 long by 0.06 mm. wide and contains 95 rows of teeth. In one 

 specimen (Plate 2, fig. 12) each row contained one central, three 

 well-developed laterals, one lateral on which no cusps could be 

 seen, and one long flat plate for an outer tooth. Another (Plate 2, 



