122 



J.A. ALLEN AND H.L. SANDERS 



D 



Fig. 33. Nuculana acuta. Internal morphology, a. anterior, b, left lateral & c, posterior views of a dissected stomach and combined style sac; d, ventral 

 view of siphonal region. See abbreviations to text-figures on p. 102. Scales = 1mm. 



broad, elongate, outlined with faint ridge, escutcheon broad, out- 

 lined by rostral ridge; umbos small, inwardly directed, anterior to 

 mid-line; antero-dorsal margin broadly concave, anterior, antero- 

 ventral, ventral and postero-ventral margins form smooth curve, 

 postero-dorsal margin raised, distally straight, proximally - poste- 

 rior to hinge plate - slightly concave, may be slightly upturned in 

 larger specimens; hinge plate elongate, relatively broad, hinge teeth 

 chevron-shaped, up to 18 teeth in each series depending on size of 

 specimen; ligament small, amphidetic, internal pear-shaped in sag- 

 ittal section, extends slightly ventral to hinge plate. The maximum 

 length of the present specimens is 9.2mm. 



Internal morphology (Figs 32, 33) 



The siphonal embayment is deep, with an elongate tentacle attached 

 to the inner right or left side. The siphons are elongate, combined 

 and except anteriorly, the ventral margins are fused. In the con- 

 tracted state the line of fusion is marked by deep ventral furrow. The 

 anterior sense organ is far anterior, situated at the point where the 

 radial ridge meets with the shell margin. Between the feeding 

 (ventral to the siphonal embayment) and the pedal gape, the inner 



folds of the ventral margin are applied to each other. In this section 

 of mantle margin approximately 30 small sensory papillae are 

 attached to each middle sensory fold in a specimen 6.5mm total 

 length. The adductor muscles are small the anterior unusually so. 

 The anterior adductor is circular in cross-section and the posterior 

 elongate-oval. 



The foot is moderately large, elongate, the sole with papillate 

 margins. The heel is not marked but there is a a large 'byssal' gland 

 present internal to the posterior limit of the sole. The gills are well- 

 developed with up to 48 alternating gill plates. The dorsal margins of 

 the left and right inner demibranchs are fused. In life the gills are a 

 bright orange-red colour. In contrast the palps are cream. The latter 

 are relatively small, elongate and dorso-ventrally narrow and, for the 

 most part, hidden under a fold of the body wall. This latter is more 

 pronounced on the right side of the body where the hind gut loop 

 meets the ventral margin of the visceral wall. Each palp has up to 24 

 ridges on the inner face. The palp proboscides are also long and 

 tapering. 



The digestive gland is bright orange in life. As in other species of 

 Nuculana, the course of the hind gut describes a single loop to the 



