106 



J.A. ALLEN AND H.L. SANDERS 



LENGTH (mm) 



Fig. 6. Neilonella salicensis. Length/frequency histogram of a sample taken from Atlantis II station 73 from the North America Basin. 



marked size groups. It is unclear whether these latter relate to annual 

 settlements or to fortuitous periodic settlements. 



Internal morphology (Figs 7 & 8) 



This was one of the few deep-sea species that were kept alive for a 

 few weeks following the processing of the sample. The external drab 

 olive/yellow of the shell is in marked contrast to the spectacularly 

 vivid array of colours of the body organs. The stomach is a bright 

 emerald green, the digestive gland a bright olive green and the gills 

 are bright red. The gill plates are particularly vivid below the frontal 

 cilia and dorso-laterally, in addition the mantle overlying the gills is 

 also red. The margins of the palps are a diffuse pink as too is a band 

 underlying the crest of the inner folds. The red and pink pigments are 

 probably a cytochrome. The blood is not red in colour and probably 

 contains a haemocyanin as do other protobranchs (Taylor, Daven- 

 port and Allen, 1995). Elsewhere the palps are pale yellow and the 

 yellow/pink effect is in marked contrast to the palp proboscides 

 which are pure white. 



Gl CV DP 



Fig. 7. Neilonella salicensis. Semidiagrammatic view of the internal 

 morphology as seen from the right side. See abbreviations to text-figures 

 on p. 102. Scale = 1mm. 



The mantle has three marginal folds of which the inner is fused at 

 the posterior end to form the siphon. Fusion is minimal, restricted to 

 the dorsal margin of the siphon (Fig. 8). Ventrally the siphonal 

 margins are unfused as too are the longitudinal ridges that mark the 

 division between the dorsal exhalent and ventral inhalent channels. 

 The gill axes attach to the inner ends of the longitudinal ridges. 

 There is a shallow siphonal embayment and attached to its inner 

 limit on the left or the right side is a siphonal tentacle which in living 

 specimens extends beyond the shell margin at distance equal to a 

 third of the shell length. Ventral to the anterior adductor muscle there 

 is an elongate anterior sense organ, which consists of a flap of tissue 

 derived from the middle sensory fold. A major ciliated rejection tract 

 is present on the inner face of the inner mantle fold. The adductor 

 muscles are relatively small, unequal in size, with the posterior the 

 smaller. The posterior muscle is round in cross-section while the 

 anterior is bean-shaped. 



The gills, with up to 26 plates, are slung from a horizontal axis 

 extending from the base of the siphon to a point about half way 

 across the body. Individual gill plates are exceptionally elongate for 

 a deep-sea protobranch. The foot is broad and the divided sole has 

 papillate margins, the anterior papillae are the more pronounced. 

 There is a small median papilla present posterior to the aperture to 

 the 'byssaF gland. The latter is extremely large. The pedal muscles 

 consist of a pair of broad posterior retractors, one on each side of the 

 hindgut anterior to the posterior adductor, and a series of four pairs 

 of anterior retractors posterior to the anterior adductor. The two 

 inner muscles of the four lie more close together than to the two 

 outer. 



The palps are relatively large with up to 27 inner ridges depending 

 on the size of the specimen. Each bears a long narrow palp probos- 

 cis. A ciliated rejection tract traverses the lateral face of the foot at 

 the junction of of the muscular and visceral parts and the posterior 

 ventral point of the palp is positioned at the posterior limit of this 

 tract in the living animal. 



The mouth is set far posterior to the anterior adductor. The 

 oesophagus curves first antero-dorsally to meet the inner face of the 

 anterior adductor then postero-dorsally to open on the anterior face 

 of the stomach. The stomach is large and slung diagonally within the 

 visceral mass. The pedal muscles form a ventral 'basket' in which 

 the stomach rests. The stomach, which is similar to that of other 



