NATURAL HISTORY OF CUMINGIA TKLLINOIDKS. 



213 



been able to recognize the older larva?. The chief difference 

 that I have observed between the young veligers of various 

 lamellibranchs is in their general appearance. Some have a 

 thicker shell than others and therefore refract light differently. 

 As is well known the veligers of all lamellibranchs are so much 

 alike that they can be distinguished, if at all, only by experts. 

 The differences are largely indefinable and are apparent only 

 after long study. 



TABLE II. 



SCHEDULE OF DEVELOPMENT AT HIGHER SUMMER TEMPERATURES, 22-23 C. 



First polar body 7 to 9 minutes 



Second polar body 27 to 30 



First cleavage 45 to 50 



Second cleavage 70 to 75 



Eggs developing at the rate shown in Table II. may reach the 

 complete veliger stage in from twenty-four to thirty hours. 

 The rate of development which Morgan gives for Cumingia 

 trochophores and veligers is considerably too slow, and his 

 statement that unfertilized centrifuged eggs are intact after 

 forty-eight hours is also erroneous. Unfertilized eggs fragment 

 within twenty-four hours. It should be noted, however, that he 

 was not particularly concerned with the rate of development 

 and these oversights do not affect the validity of the conclusions 

 he draws. 



TABLE III. 



MEASUREMENTS AT VARIOUS AGES. 



Egg .061 to .065 mm. average .062 to .063. 



Blastula same as above. 



Trochophore same as above but slightly elongated anteroposteriorly. 



Two day veliger .074 x .090 x .055 mm. to .08 x .091 x .055 mm. 



Ten day veliger .09 x .10 mm. 



Two weeks veliger .095 x .105 mm. 



Three weeks Cumingia metamorphosed .09 x .105 mm. to .097 x .155 mm. 



Four to five months (size attained before first winter.), 6 x 5 mm. to io x 8 mm. 



One year, n x 8 mm. to 13 x 9 mm. (sexually mature). 



Two years, 14 x n mm. to 16 x n mm. 



Three years, 17 x 12 mm. to 17 x 13 mm. 



Four years, 18x12 mm. to 19.5 x 14 mm. (Largest specimens found). 



Experiments are in progress to learn the normal length of lite 

 of Cumingia. It appears that they are approximately half 



