. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 512 to the exterior in the position formerly occupied by the blastopore. The stomodseum is often attached to the anterior, and to part of the ventral portions of the foot, but the posterior end lies free between the mantle-lobes, sometimes almost, if not quite, in contact with the test, sometimes some distance from it. The anterior end of the mid- gut has enlarged to form the stomach, which communicates with the pouches of the digestive glands. A large space has been left dorsal to the intestine. The cerebral, eg, pedal, pg,
. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 512 to the exterior in the position formerly occupied by the blastopore. The stomodseum is often attached to the anterior, and to part of the ventral portions of the foot, but the posterior end lies free between the mantle-lobes, sometimes almost, if not quite, in contact with the test, sometimes some distance from it. The anterior end of the mid- gut has enlarged to form the stomach, which communicates with the pouches of the digestive glands. A large space has been left dorsal to the intestine. The cerebral, eg, pedal, pg, and visceral, vg, gauglia, with their commissures, have been formed. The otocysts, ot, have apparently been entirely closed off, and each contains an otolith, which stains deeply with hsematoxylin and, a little later, plainly shows concentric structure. As the otocysts have never been open to the exterior, development having taken place inside a closed test, the otoliths cannot be foreign particles. The only places where the test is attached to the body of the embryo lying inside it, are around, the blastopore, the apical plate, and the opening of the pouch that extends to the cerebral ganglia. The embryos still swim rapidly, but have periods of rest. The early embryos of N. delphinodonta do not develop well out- side of the egg cases, and the age of embryos that correspond in development to the embryos of Yoldia just described, is not accurately known. They develop much more slowly than the embryos of either of the other species, and it seems prob- able that a corre- sponding stage is reached in about ten days or two weeks. The embryos of this species are much larger than the em- bryos of either of the other species. They present more rounded outlines, and are less com- pressed laterally, ^std Fig. 17. Reconstruction of an embryo of Nucula delphinodonta, at a stage just before the test is cast off. The embryo is seen from the right side. Specimens are 0,32 mm long, aa anterior
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