. Journal of morphology. Fig. 7. — Slightly older Gastrula of 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, approa
. Journal of morphology. Fig. 7. — Slightly older Gastrula of 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
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