DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



35 



posterior margin, the mantle edge is little modified and 

 comprises three marginal lobes. The middle sensory lobe 

 forms a frill while the inner muscular fold is thickened, being 

 approximately twice the thickness of the general mantle 

 epithelium and is relatively broad. Posteriorly combined 

 exhalent and inhalent siphons are formed by fusion of the 

 opposing inner muscular lobes. The inhalent siphon is shorter 

 than the exhalent siphon and remains open ventrally. The 

 siphonal wall is relatively thin. A narrow band of longitudinal 

 muscles lies immediately internal to the basal membrane of 

 the outer and inner epithelia. Internal to the longitudinal 

 fibres are circular and transverse muscle fibres with sub- 

 epithelial gland cells scattered throughout the layer. Between 

 the exhalent and inhalent siphons, there is a pair of haemo- 

 coelic channels within the muscle layer (Fig. 48a). The 

 proximal end of the gill axes join laterally at the junction 

 between the siphons. The gill axes probably act as a channel 

 to guide faecal rods into the lumen of the exhalent siphon. 

 The anus lies immediately dorsal to the inner margins of the 

 siphon. 



The siphonal tentacle, is developed from the middle sen- 

 sory lobe of the mantle and originates from a pocket in the 

 siphonal embayment at the base of the siphon, usually on the 

 left side. It consists of an elongate finely-tapering cone which 

 in transverse section comprises a large central nerve sur- 

 rounded by several muscle fibres (Fig. 48b). These in turn are 

 surrounded by a layer of connective tissue with gland cells 

 interspersed throughout and a layer of epithelial cells with 

 fine granular contents. 



Ventral to the siphon is a broad, deep, specialized area of 

 the mantle - the feeding aperture. Ventral to the inhalent 

 aperture there is an inner secondary muscular fold. Peripheral 

 •to this the inner muscular layer is enlarged both in thickness 

 and width. In preserved specimens this area is highly convo- 

 luted. In life the muscular and sensory layers are probably 

 extended beyond the shell margin as a flap on each side. The 

 epithelial cells in this region are densely ciliated (Fig. 48a). 



The adductor muscles are unequal in size with the 'quick' 

 and 'catch' parts obvious. The posterior adductor is round in 



outline. The anterior is approximately twice the size of the 

 posterior and crescent-shaped. At the mantle margin ventral 

 to the anterior adductor muscle there is a well-developed 

 anterior sense organ. This is derived from the middle sensory 

 fold which is greatly extended to form two flaps. The epithe- 

 lial cells of the outer flap are ciliated. Internal to this the 

 epithelium is glandular (Fig. 48c). Underlying this epithelium 

 is a thick layer of connective tissue containing the pallial 

 nerve. 



The gills lie parallel to the postero-dorsal shell margin. 

 They are well-developed with up to 22 alternating gill plates 

 (the number relates to the size of the individual). 



The labial palps are moderately large and, also depending 

 on the size of the animal, have up to 24 ridges on their inner 

 face. The palp proboscides are relatively slender. The palps 

 are wedge-shaped and extend 1/4-1/2 way across the body. 

 The mouth is set posterior to the anterior adductor muscle. 



The foot is well-developed, and of typical nuculanid form. 

 The divided sole is elongate with papillate edges. At the tips 

 of the papillae, lying between muscle fibres are glandular 

 cells with ducts to the surface of the foot. Secretions from 

 these may be used to lubricate the movement of the foot 

 through the sediment. A large well-developed byssal gland is 

 present in the heel of the foot (Fig. 49a). The byssal gland is 

 spherical, composed of large, hyaline cells, surrounded by 

 muscle fibres. It opens medially at the posterior limit of the 

 divided sole and several glandular epithelial cells surround 

 the opening. A very small medial papilla lies posterior to the 

 opening at the heel of the foot. Histochemical tests have been 

 carried out to identify type secretion from the gland. Tests 

 did not confirm any protein component but this could have 

 been masked by other muco-substances. The main compo- 

 nent at the centre of the byssal gland appears to be a 

 keratin-sulphate (PAS-ve with no glycol groups, and carboxyl 

 groups). It has the character of cartilage. (Secretions from the 

 gland cells in the foot do, however, possess carboyxyl 

 groups). 



The posterior pedal retractor muscles comprise a thick, 

 wide, strap that inserts on the shell on either side of the hind 

 gut anterior to the posterior adductor muscle. A small 

 postero-lateral pedal retractor muscle is present on either 

 side of the stomach. Three major pairs of anterior pedal 

 retractor muscles pass from the neck of the foot anteriorly to 

 insert posterior to the anterior adductor muscle on either side 



PSA 



ig. 48 Yoldiella lata. Transverse sections through a, the inhalent, 

 exhalent and feeding apertures; b, the base of the siphonal 

 tentacle; c, the anterior sense organ of a specimen from Sta. BG 

 III DS 49 West European Basin. (Scales = 0.1 mm). 



Fig. 49 Yoldiella lata, a, transverse section through the heel and 

 'byssal' gland of the foot and b, right and left lateral external 

 views of the stomach and style sac of specimens from Sta. Polygas 

 DS 26 West European Basin. (Scales = 0.1 mm). 



