DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



43 



50 



40-, 



W\L 



30 J 

 80 



H1L 





60 

 60 

 50^ 



PLITL 



Length (mm) 



Fig. 62 Yoldiella inconspkua inconspicua . Variation in the ratios 

 of height H/L, width W/L and postero-umbonal length PL/TL to 

 length against length of a sample from Sta. INCAL CP 08 West 

 European Basin. 



vertically below the anterior adductor muscle. A large sipho- 

 nal tentacle is usually attached on the right side of the 

 innermost wall of the siphonal embayment, but was recorded 

 on the left in a few specimens. Siphons are combined and 

 form a wide tube with a single lumen. The feeding aperture is 

 not particularly well-developed, being represented in Y. 

 inconspicua by two small flaps ventral to the combined 

 siphon. The inner muscular layer is not expanded anterior to 

 these, as it is in some other Yoldiella species. The adductor 

 muscles are large and approximately equal in size. The 

 posterior muscle is circular and the anterior more 'crescent- 

 shaped'. 

 The gills are well-developed and filaments number from 



Fig. 63 Yoldiella inconspicua inconspicua. Lateral view from the 

 right side of the internal morphology of a specimen from Sta. 72, 

 North Atlantic Basin. (Scale = 1.0 mm). For identification of the 

 parts see Fig. 34. 



2 3 



Length (mm) 



Fig. 64 Yoldiella inconspicua inconspicua. Length frequency 

 histograms of two samples from a, Sta. 72 North America Basin 

 and b, Sta. INCAL CP 08 West European Basin. 



12-23, the latter in a large individual 3.55 mm in length. The 

 distal gill filaments lie close to the siphon and the gill axis 

 attaches to the siphon ventro-laterally. The labial palps are 

 moderately large with between 12-22 ridges on the internal 

 face. The palp proboscides are long and thin. The cerebral 

 ganglia are large, the visceral ganglia are moderate in size and 

 the pedal ganglia are large and elongate oval in shape. They 

 lie more dorsal in position in the foot than in most other 

 Yoldiella species. The foot is divided and fringed with papil- 

 lae and has a large byssal gland in the heel. 



The oesophagus is distended and wide in diameter. The 

 longitudinal plane of the stomach lies at an angle to the 

 vertical sagittal plane, and anteriorly is displaced to the right 

 while posteriorly it lies to the left. The stomach is large but 

 somewhat laterally flattened. The style sac is small. The first 

 section of the hind gut which lies within the foot is wide in 

 section from thence it curves dorsally parallel to the posterior 

 margin of the foot. At its dorsal limit of its course there are 

 two small kinks to the left of the body (possibly a result of 

 contraction in preserved specimens) before it crosses to the 

 right side to form a single anterior loop the outline of which is 

 characteristic of the species. The diameter of the gut appears 

 to vary somewhat between populations, western Atlantic 

 specimens appear to have a more slender hindgut although 

 the dimensions fall within the overall range of eastern Atlan- 

 tic specimens. A typhlosole is present along the whole length 

 of the gut. A considerable amount of food material is 

 frequently present in the left hand digestive diverticulum. 

 The digestive diverticula extend well posterior within the 



