8 



(Plate 2, fig. 6), and a flat dorsal disk appears which later supports 

 the shell. Within a few hours a strong muscular motion may be 

 seen just behind the mouth which has taken the form of a circular 

 hole. This motion is internal and is a slow forward and upward 

 movement of the buccal mass and a sudden backward and down- 

 ward withdrawing of the same. It is identical wdth the feeding 

 motion of the radula and buccal mass of the mature animal. 

 INIinute, light-brown eye-spots appear during the eighth or ninth 

 day becoming gradually black within 24 hours. At the same time 

 a rhythmic pulsation of the pulmonary chamber may be seen. This 

 chamber (Plate 2, figs. 8, 9) is situated dorsally on the middle of 

 the left side. Ordinarily the pulsations occur at the rate of 50 

 to 60 per minute, but embryos were observed with a pulmonary 

 pulsation as high as 120 per minute. In these -cases, high tem- 

 perature or lack of fresh water appears to be the cause for the 

 high rate of pulsation; fresh cool water almost immediately re- 

 stores it to normal. 



By the eighth or ninth day the embryos are twisting and turn- 

 ing in every direction, while the buccal mass is being constantly 

 pushed Out against the walls of the capsule and quickly with- 

 drawn. This constant prodding of the twisting and turning em- 

 bryos, assisted by the action of the buccal mass, soon has the effect 

 of weakening the capsule-wall. It loses its rigidity, bending more 

 and more under the struggles of the embryo, and finally ruptures, 

 allowing the embryo to escape into the larger cavity of the egg- 

 case. In those egg-cases containing several capsules, each with 

 normally developing eggs, all of the embryos have escaped from 

 their respective capsules by the tenth day, and continue their 

 development in the common chamber of the egg-case. In some 

 egg-cases one or more of the capsules may contain eggs which have 

 not developed. In this event the capsule- wall retains its rigidity 

 and only those capsules containing normally developed embryos 

 are ruptured, for the struggles of the embryo in an adjoining 

 capsule have apparently no effect on the capsule-wall of an unde- 

 veloped egg. The embryos continue to grow and develop for a 

 week or ten days in the egg-case, after having torn down the 

 capsule walls and converted the egg-case into one common cham- 

 ber. Eventually through constant prodding by the buccal mass, 

 rasping with the rudimentary radula and by the muscular motion 

 of the body, an opening is made in the egg-case where it is fastened 

 to its base, and in a few moments all the inmates have escaped. 

 The embryos from one case deposited during the interval between 



