DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



49 



Fig. 71 Yoldiella 

 curta. a, lateral 

 external and 

 internal view of 

 left valve of the 

 type specimen of 

 Y. curta (USNM 

 No.38457) and b, 

 the lateral 

 external and 

 internal view of 

 the right valve of 

 the type specimen 

 of Y. miniscula 

 (USNM No. 

 38415). Specimens 

 from the U.S. Fish 

 Commission Sta. 

 2084 North 

 America Basin. 

 (Scale =1.0 mm). 



mantle cavity. The outer and inner demibranchs are attached 

 to the mantle and body respectively by tissue junctions. 

 Depending on the size of the individual there are 11-16 

 alternating gill plates. The labial palps are moderately large, 

 extending approximately 1/3 across the body and have 

 between 13-19 wide ridges on their inner faces. 



The foot is large and well-developed. A large byssal gland 

 is present, with little obvious internal structure as seen in 

 whole mounts. At the point where the gland opens to the 

 exterior the surrounding tissue contains acidophilic gland and 

 basiphilic cells. As in other species of Yoldiella there is a 

 broad band of posterior retractor muscles. The latero- 

 posterior retractor muscle is inserted ventrally on lateral body 

 wall. There are three major pairs of anterior pedal retractor 

 muscles which, from the neck of the foot, pass to either side 

 of the oesophagus and insert on the shell posterior to the 

 anterior adductor muscle. Another smaller pair of anterior 

 retractor muscles insert immediately posterior to the anterior 

 adductor and pass within the ventral visceral margin at 

 epithelia to the region of the mouth (Fig. 76b). Posterior to 

 these a second pair of muscles pass vertically from the dorsal 

 shell margin to the labial palps. 



The cerebral ganglia are relatively large, as are the 

 elongate-oval pedal ganglia. The visceral ganglia are smaller, 

 cylindrical and lie close to the antero-ventral margin of the 

 posterior adductor muscle. 



The mouth is a short distance posterior to the anterior 

 adductor muscle. The oesophagus curves dorsally and anteri- 

 orly before turning posteriorly to enter the stomach approxi- 

 mately midway on the left side of the anterior face. No 

 sorting ridges are visible externally on the right side of the 

 stomach. There is a prominent gastric tooth at the left 

 antero-dorsal edge of the gastric shield. The latter extends 

 posteriorly over the left, the dorsal and part of the right wall. 

 The style sac is small, narrow and penetrates the foot to level 

 of pedal ganglia. The digestive diverticula are extensive 

 occurring on the right and left sides of the body and anterior 

 to the stomch. Fine material was observed in the lumen of the 

 left digestive diverticulum. A duct from the left side opens 

 into the stomach below the gastric tooth. The course of the 

 two other ducts are less clear, but from the evidence of other 

 species they probably open into the stomach anteriorly close 

 to the oesophagus. The hind gut sweeps across the ventral 



Fig. 72 Yoldiella curta. a, lateral view from right side of shell, 

 detail of the hinge-plate of a right valve and dorsal view of a shell 

 from U.S. Fish Commission Sta. 2073 North America Basin, 

 compared with b, a lateral view of a shell from the left side and 

 detail of the hinge-plate of a right valve from Sta. Polygas DS 25 

 West European Basin. (Scale = 1.0 mm). 



right side of the viscera to the anterior adductor muscle, 

 where it crosses to the left side of the body to form a single 

 loop before returning to the right alongside the outward 



