72 



LILLIE. 



[Vol. X. 



ment in mind in such considerations. The accompanying text 

 figures (6 to 9) require but little explanation ; they show that 

 there is no real difficulty in recognizing the homologous areas 

 in Unio, Cyclas, and Teredo. The latter is one of the most 

 typical of the marine veligers ; that is to say, approaches most 

 nearly the trochophore in its structure. 



Of the typical trochophore organs, the apical plate with its 

 tuft of cilia, the praeoral and postoral rows of cilia and the 

 head kidney are missing in Unio. These are of course among the 

 most characteristic organs of the trochophore and most essential 

 to the free life of the larva. It is these organs which always 

 degenerate more or less subsequent to the giving up of the 



Fig. 10. — Young Larva of Unio. 

 N.A. Neural Axis. D.A. Dorsal Axis. 



Fig. II.— Teredo (after Hatschek). 

 N.A. Neural Axis. D.A. Dorsal Axis. 



free life. A complete series can be traced through the various 

 degrees of degeneracy of the organs in question to their com- 

 plete absence in the Unionidae, where we can recognize only 

 the homologous areas. The swollen cells of the head vesicle 

 are the only remaining differentiation of the apical area which 

 can be interpreted as rudimentary trochophore organs. 



It is important to notice that in the veliger stage of all 

 Mollusca the long axes of the shell or shell-gland and of the 

 foot, which in the adult are parallel, are inclined at an angle of 

 nearly 90° to one another. The figures 10, 11, and 12 in the 

 text illustrate this in Anodonta, Ostrea, and Teredo. Even the 

 most cursory examination of Gasteropod larvae will show that 

 the same thing occurs there. This is due to the fact that the 

 dorsal and ventral surfaces of the trunk are independently 



