. Journal of morphology. Fig. 8. — Cyclas (after Stauffacher) Bp. Head-vesicle, Oral plate. Ventral Mesoblast. Teredo (after Hatsthek). oes. Oesophagus, Larval Mesoblast. 72 LILLIE. [Vol. X. ment in mind in such considerations. The accompanying textfigures (6 to 9) require but little explanation ; they show thatthere is no real difficulty in recognizing the homologous areasin Unio, Cyclas, and Teredo. The latter is one of the mosttypical of the marine veligers ; that is to say, approaches mostnearly the trochophore in its struc
. Journal of morphology. Fig. 8. — Cyclas (after Stauffacher) Bp. Head-vesicle, Oral plate. Ventral Mesoblast. Teredo (after Hatsthek). oes. Oesophagus, Larval Mesoblast. 72 LILLIE. [Vol. X. ment in mind in such considerations. The accompanying textfigures (6 to 9) require but little explanation ; they show thatthere is no real difficulty in recognizing the homologous areasin Unio, Cyclas, and Teredo. The latter is one of the mosttypical of the marine veligers ; that is to say, approaches mostnearly the trochophore in its structure. Of the typical trochophore organs, the apical plate with itstuft of cilia, the praeoral and postoral rows of cilia and thehead kidney are missing in Unio. These are of course among themost characteristic organs of the trochophore and most essentialto the free life of the larva. It is these organs which alwaysdegenerate more or less subsequent to the giving up of the. Fig. 10. —Young Larva of Neural Axis. Dorsal Axis. Fig. II.—Teredo (after Hatschek). Neural Axis. Dorsal Axis. free life. A complete series can be traced through the variousdegrees of degeneracy of the organs in question to their com-plete absence in the Unionidae, where we can recognize onlythe homologous areas. The swollen cells of the head vesicleare the only remaining differentiation of the apical area whichcan be interpreted as rudimentary trochophore organs. It is important to notice that in the veliger stage of allMollusca the long axes of the shell or shell-gland and of thefoot, which in the adult are parallel, are inclined at an angle ofnearly 90° to one another. The figures 10, 11, and 12 in thetext illustrate this in Anodonta, Ostrea, and Teredo. Even themost cursory examination of Gasteropod larvae will show thatthe same thing occurs there. This is due to the fact that thedorsal and ventral surfaces of the trunk are independently No. I.] THE EM
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