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 adductor muscle, ap apical plate, int intestine, rl right lobe of the 

 digestive gland, std stomodseum. 



- -ap 



