. Bulletin of the Natural History Museum Zoology. DEEP-SEA PROTOBRANCHIA (BIVALVIA) 19 heavily ciliated throughout. The adductor muscles are rela- tively large and unequal in size. The posterior muscle is oval in outline and is approximately 1/2 to 1/3 the size of the anterior muscle. The gills are well-developed with up to 16 gill filaments depending on the size of the individual. The number of ridges on the inner surface of the labial palp varies between 12-17. The palps extend over approximately half the body width and have long, moderately thick palp proboscides. The visceral and cerebral
. Bulletin of the Natural History Museum Zoology. DEEP-SEA PROTOBRANCHIA (BIVALVIA) 19 heavily ciliated throughout. The adductor muscles are rela- tively large and unequal in size. The posterior muscle is oval in outline and is approximately 1/2 to 1/3 the size of the anterior muscle. The gills are well-developed with up to 16 gill filaments depending on the size of the individual. The number of ridges on the inner surface of the labial palp varies between 12-17. The palps extend over approximately half the body width and have long, moderately thick palp proboscides. The visceral and cerebral ganglia are elongate and each large pedal ganglia has a large statocyst dorsal to it. The foot is large and directed anteriorly, with well-developed retractor muscles. There is a moderately large byssal gland in the heel. The oesphagus is wide and opens into a voluminous stomach. The style sac penetrates into the lower portion of the foot. The hind gut forms a single loop on the right side of the body and has a typhlosole along its entire length. The digestive diver- ticula, as in other species, are situated one to the right and two to the left of the stomach. The ducts to the left hand diverticula enter the stomach close together, ventral to the gastric shield one posterior to the other. The kidney is small. Gonads overlie the lateral and dorsal sides of the viscera and the sexes are separate. Y. obesa and Y. lucida differ as follows:- (1) The ventral margin of Y. obesa is slightly deeper and more rounded. (2) The anterior adductor of Y. obesa is relatively smaller when compared with a specimen of Y. lucida the same size. (3) The internal ligament of Y. obesa is smaller, shorter and more rounded. (4) The umbo is more anterior in Y. obesa and slightly larger and more prominent. (5) The posterior margin is more rostrate and tapered in Y. obesa and Fig. 11 Yoldiella obesa incala. Lateral view of shells from the right side to show change in shape with increasing size, and det
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